diff --git a/.travis.yml b/.travis.yml
index a563ed67c0..471c8bd9f6 100644
--- a/.travis.yml
+++ b/.travis.yml
@@ -60,6 +60,10 @@ install:
       fi
     fi
 
+  # Perceptual image hashing (TBD: push recipe to conda-forge!)
+  - conda install pip
+  - pip install imagehash
+
   - PREFIX=$HOME/miniconda/envs/$ENV_NAME
 
   # Output debug info
diff --git a/docs/iris/example_tests/extest_util.py b/docs/iris/example_tests/extest_util.py
index 77b5493ef5..3d9017b6ee 100644
--- a/docs/iris/example_tests/extest_util.py
+++ b/docs/iris/example_tests/extest_util.py
@@ -35,7 +35,6 @@
 from iris._deprecation import IrisDeprecation
 import iris.plot as iplt
 import iris.quickplot as qplt
-from iris.tests import _DEFAULT_IMAGE_TOLERANCE
 
 
 EXAMPLE_DIRECTORY = os.path.join(os.path.dirname(os.path.dirname(__file__)),
@@ -60,7 +59,7 @@ def add_examples_to_path():
 
 
 @contextlib.contextmanager
-def show_replaced_by_check_graphic(test_case, tol=_DEFAULT_IMAGE_TOLERANCE):
+def show_replaced_by_check_graphic(test_case):
     """
     Creates a context manager which can be used to replace the functionality
     of matplotlib.pyplot.show with a function which calls the check_graphic
@@ -69,7 +68,7 @@ def show_replaced_by_check_graphic(test_case, tol=_DEFAULT_IMAGE_TOLERANCE):
     """
     def replacement_show():
         # form a closure on test_case and tolerance
-        test_case.check_graphic(tol=tol)
+        test_case.check_graphic()
 
     orig_show = plt.show
     plt.show = iplt.show = qplt.show = replacement_show
diff --git a/docs/iris/example_tests/test_atlantic_profiles.py b/docs/iris/example_tests/test_atlantic_profiles.py
index 5592663a56..63d6b7b754 100644
--- a/docs/iris/example_tests/test_atlantic_profiles.py
+++ b/docs/iris/example_tests/test_atlantic_profiles.py
@@ -33,7 +33,7 @@ def test_atlantic_profiles(self):
         with fail_any_deprecation_warnings():
             with add_examples_to_path():
                 import atlantic_profiles
-            with show_replaced_by_check_graphic(self, tol=14.0):
+            with show_replaced_by_check_graphic(self):
                 atlantic_profiles.main()
 
 
diff --git a/lib/iris/plot.py b/lib/iris/plot.py
index 025e32ac42..5b4f2989db 100644
--- a/lib/iris/plot.py
+++ b/lib/iris/plot.py
@@ -27,6 +27,8 @@
 
 import collections
 import datetime
+from functools import wraps
+import threading
 
 import cartopy.crs as ccrs
 import cartopy.mpl.geoaxes
@@ -52,9 +54,28 @@
 # Cynthia Brewer citation text.
 BREWER_CITE = 'Colours based on ColorBrewer.org'
 
-
 PlotDefn = collections.namedtuple('PlotDefn', ('coords', 'transpose'))
 
+# Threading reentrant lock to ensure thread-safe plotting.
+_lock = threading.RLock()
+
+
+def _locker(func):
+    """
+    Decorator that ensures a thread-safe atomic operation is
+    performed by the decorated function.
+
+    Uses a shared threading reentrant lock to provide thread-safe
+    plotting by public API functions.
+
+    """
+    @wraps(func)
+    def decorated_func(*args, **kwargs):
+        with _lock:
+            result = func(*args, **kwargs)
+        return result
+    return decorated_func
+
 
 def _get_plot_defn_custom_coords_picked(cube, coords, mode, ndims=2):
     def names(coords):
@@ -665,6 +686,7 @@ def _map_common(draw_method_name, arg_func, mode, cube, plot_defn,
     return plotfn(*new_args, **kwargs)
 
 
+@_locker
 def contour(cube, *args, **kwargs):
     """
     Draws contour lines based on the given Cube.
@@ -689,6 +711,7 @@ def contour(cube, *args, **kwargs):
     return result
 
 
+@_locker
 def contourf(cube, *args, **kwargs):
     """
     Draws filled contours based on the given Cube.
@@ -815,6 +838,7 @@ def _fill_orography(cube, coords, mode, vert_plot, horiz_plot, style_args):
     return result
 
 
+@_locker
 def orography_at_bounds(cube, facecolor='#888888', coords=None, axes=None):
     """Plots orography defined at cell boundaries from the given Cube."""
 
@@ -845,6 +869,7 @@ def horiz_plot(v_coord, orography, style_args):
                            horiz_plot, style_args)
 
 
+@_locker
 def orography_at_points(cube, facecolor='#888888', coords=None, axes=None):
     """Plots orography defined at sample points from the given Cube."""
 
@@ -866,6 +891,7 @@ def horiz_plot(v_coord, orography, style_args):
                            horiz_plot, style_args)
 
 
+@_locker
 def outline(cube, coords=None, color='k', linewidth=None, axes=None):
     """
     Draws cell outlines based on the given Cube.
@@ -903,6 +929,7 @@ def outline(cube, coords=None, color='k', linewidth=None, axes=None):
     return result
 
 
+@_locker
 def pcolor(cube, *args, **kwargs):
     """
     Draws a pseudocolor plot based on the given Cube.
@@ -929,6 +956,7 @@ def pcolor(cube, *args, **kwargs):
     return result
 
 
+@_locker
 def pcolormesh(cube, *args, **kwargs):
     """
     Draws a pseudocolor plot based on the given Cube.
@@ -953,6 +981,7 @@ def pcolormesh(cube, *args, **kwargs):
     return result
 
 
+@_locker
 def points(cube, *args, **kwargs):
     """
     Draws sample point positions based on the given Cube.
@@ -980,6 +1009,7 @@ def _scatter_args(u, v, data, *args, **kwargs):
                                 *args, **kwargs)
 
 
+@_locker
 def plot(*args, **kwargs):
     """
     Draws a line plot based on the given cube(s) or coordinate(s).
@@ -1024,6 +1054,7 @@ def plot(*args, **kwargs):
     return _draw_1d_from_points('plot', _plot_args, *args, **kwargs)
 
 
+@_locker
 def scatter(x, y, *args, **kwargs):
     """
     Draws a scatter plot based on the given cube(s) or coordinate(s).
@@ -1059,6 +1090,7 @@ def scatter(x, y, *args, **kwargs):
 show = plt.show
 
 
+@_locker
 def symbols(x, y, symbols, size, axes=None, units='inches'):
     """
     Draws fixed-size symbols.
@@ -1122,6 +1154,7 @@ def symbols(x, y, symbols, size, axes=None, units='inches'):
     axes.autoscale_view()
 
 
+@_locker
 def citation(text, figure=None, axes=None):
     """
     Add a text citation to a plot.
diff --git a/lib/iris/tests/__init__.py b/lib/iris/tests/__init__.py
index ea25e6725f..d732a70072 100644
--- a/lib/iris/tests/__init__.py
+++ b/lib/iris/tests/__init__.py
@@ -41,7 +41,6 @@
 import filecmp
 import functools
 import gzip
-import hashlib
 import inspect
 import json
 import io
@@ -52,6 +51,7 @@
 import subprocess
 import sys
 import unittest
+import threading
 import warnings
 import xml.dom.minidom
 import zlib
@@ -116,6 +116,10 @@
 
 #: Basepath for test results.
 _RESULT_PATH = os.path.join(os.path.dirname(__file__), 'results')
+#: Default perceptual hash size.
+_HASH_SIZE = 16
+#: Default maximum perceptual hash hamming distance.
+_HAMMING_DISTANCE = 2
 
 if '--data-files-used' in sys.argv:
     sys.argv.remove('--data-files-used')
@@ -143,7 +147,8 @@
     plt.switch_backend('tkagg')
     _DISPLAY_FIGURES = True
 
-_DEFAULT_IMAGE_TOLERANCE = 10.0
+# Threading non re-entrant blocking lock to ensure thread-safe plotting.
+_lock = threading.Lock()
 
 
 def main():
@@ -661,7 +666,7 @@ def _ensure_folder(self, path):
             logger.warning('Creating folder: %s', dir_path)
             os.makedirs(dir_path)
 
-    def _assert_graphic(self):
+    def _assert_graphic(self, tol=_HAMMING_DISTANCE):
         """
         Check the hash of the current matplotlib figure matches the expected
         image hash for the current graphic test.
@@ -672,10 +677,12 @@ def _assert_graphic(self):
         output directory, and the imagerepo.json file being updated.
 
         """
+        import imagehash
+        from PIL import Image
+
         dev_mode = os.environ.get('IRIS_TEST_CREATE_MISSING')
         unique_id = self._unique_id()
-        repo_fname = os.path.join(os.path.dirname(__file__),
-                                  'results', 'imagerepo.json')
+        repo_fname = os.path.join(_RESULT_PATH, 'imagerepo.json')
         with open(repo_fname, 'rb') as fi:
             repo = json.load(codecs.getreader('utf-8')(fi))
 
@@ -695,78 +702,85 @@ def _assert_graphic(self):
             result_fname = os.path.join(image_output_directory,
                                         'result-' + unique_id + '.png')
 
-            if not os.path.isdir(os.path.dirname(result_fname)):
+            if not os.path.isdir(image_output_directory):
                 # Handle race-condition where the directories are
                 # created sometime between the check above and the
                 # creation attempt below.
                 try:
-                    os.makedirs(os.path.dirname(result_fname))
+                    os.makedirs(image_output_directory)
                 except OSError as err:
                     # Don't care about "File exists"
                     if err.errno != 17:
                         raise
 
-            def _save_figure_hash():
-                plt.gcf().savefig(result_fname)
-                # Determine the test result image hash using sha1.
-                with open(result_fname, 'rb') as fi:
-                    sha1 = hashlib.sha1(fi.read())
-                return sha1
-
             def _create_missing():
-                fname = sha1.hexdigest() + '.png'
-                base_uri = ('https://scitools.github.io/test-images-scitools/'
-                            'image_files/{}')
+                fname = '{}.png'.format(phash)
+                base_uri = ('https://scitools.github.io/test-iris-imagehash/'
+                            'images/{}')
                 uri = base_uri.format(fname)
                 hash_fname = os.path.join(image_output_directory, fname)
                 uris = repo.setdefault(unique_id, [])
                 uris.append(uri)
                 print('Creating image file: {}'.format(hash_fname))
-                os.rename(result_fname, hash_fname)
+                figure.savefig(hash_fname)
                 msg = 'Creating imagerepo entry: {} -> {}'
                 print(msg.format(unique_id, uri))
                 with open(repo_fname, 'wb') as fo:
                     json.dump(repo, codecs.getwriter('utf-8')(fo), indent=4,
                               sort_keys=True)
 
-            sha1 = _save_figure_hash()
+            # TBD: Push this fix to imagehash (done!)
+            # See https://github.com/JohannesBuchner/imagehash/pull/31
+            # Now need this imagehash/master pushed to pypi ...
+            def _hex_to_hash(hexstr, hash_size=_HASH_SIZE):
+                l = []
+                count = hash_size * (hash_size // 4)
+                if len(hexstr) != count:
+                    emsg = 'Expected hex string size of {}.'
+                    raise ValueError(emsg.format(count))
+                for i in range(count // 2):
+                    h = hexstr[i*2:i*2+2]
+                    v = int("0x" + h, 16)
+                    l.append([v & 2**i > 0 for i in range(8)])
+                return imagehash.ImageHash(np.array(l))
+
+            # Calculate the test result perceptual image hash.
+            buffer = io.BytesIO()
+            figure = plt.gcf()
+            figure.savefig(buffer, format='png')
+            buffer.seek(0)
+            phash = imagehash.phash(Image.open(buffer), hash_size=_HASH_SIZE)
 
             if unique_id not in repo:
                 if dev_mode:
                     _create_missing()
                 else:
+                    figure.savefig(result_fname)
                     emsg = 'Missing image test result: {}.'
                     raise ValueError(emsg.format(unique_id))
             else:
                 uris = repo[unique_id]
-                # Cherry-pick the registered expected hashes from the
-                # test case uri/s.
-                expected = [os.path.splitext(os.path.basename(uri))[0]
+                # Create the expected perceptual image hashes from the uris.
+                expected = [_hex_to_hash(os.path.splitext(os.path.basename(uri))[0])
                             for uri in uris]
 
-                if sha1.hexdigest() not in expected:
-                    # This can be an accidental failure, unusual, but it occurs
-                    # https://github.com/SciTools/iris/issues/2195
-                    # retry once, in case it passes second time round.
-                    sha1 = _save_figure_hash()
+                # Calculate the hamming distance vector for the result hash.
+                distances = [e - phash for e in expected]
 
-                if sha1.hexdigest() not in expected:
+                if np.all([hd > tol for hd in distances]):
                     if dev_mode:
                         _create_missing()
                     else:
-                        emsg = 'Actual SHA1 {} not in expected {} for test {}.'
-                        emsg = emsg.format(sha1.hexdigest(), expected,
-                                           unique_id)
+                        figure.savefig(result_fname)
+                        msg = ('Bad phash {} with hamming distance {} '
+                               'for test {}.')
+                        msg = msg.format(phash, distances, unique_id)
                         if _DISPLAY_FIGURES:
-                            print('Image comparison would have failed. '
-                                  'Message: %s' % emsg)
+                            emsg = 'Image comparion would have failed: {}'
+                            print(emsg.format(msg))
                         else:
-                            raise ValueError('Image comparison failed. '
-                                             'Message: {}'.format(emsg))
-                else:
-                    # There is no difference between the actual and expected
-                    # result, so remove the actual result file.
-                    os.remove(result_fname)
+                            emsg = 'Image comparison failed: {}'
+                            raise ValueError(emsg.format(msg))
 
             if _DISPLAY_FIGURES:
                 plt.show()
@@ -774,14 +788,13 @@ def _create_missing():
         finally:
             plt.close()
 
-    def check_graphic(self, tol=None):
+    def check_graphic(self):
         """
         Checks that the image hash for the current matplotlib figure matches
         the expected image hash for the current test.
 
         """
-        fname = os.path.join(os.path.dirname(__file__),
-                             'results', 'imagerepo.lock')
+        fname = os.path.join(_RESULT_PATH, 'imagerepo.lock')
         lock = filelock.FileLock(fname)
         # The imagerepo.json file is a critical resource, so ensure thread
         # safe read/write behaviour via platform independent file locking.
@@ -850,7 +863,13 @@ def assertArrayShapeStats(self, result, shape, mean, std_dev):
 
 class GraphicsTest(IrisTest):
 
+    # nose directive: dispatch tests concurrently.
+    _multiprocess_can_split_ = True
+
     def setUp(self):
+        # Acquire threading non re-entrant blocking lock to ensure
+        # thread-safe plotting.
+        _lock.acquire()
         # Make sure we have no unclosed plots from previous tests before
         # generating this one.
         if MPL_AVAILABLE:
@@ -861,10 +880,11 @@ def tearDown(self):
         # in an odd state, so we make sure it's been disposed of.
         if MPL_AVAILABLE:
             plt.close('all')
+        # Release the non re-entrant blocking lock.
+        _lock.release()
 
 
 class TestGribMessage(IrisTest):
-
     def assertGribMessageContents(self, filename, contents):
         """
         Evaluate whether all messages in a GRIB2 file contain the provided
diff --git a/lib/iris/tests/experimental/test_animate.py b/lib/iris/tests/experimental/test_animate.py
index 95f7804af4..2342ecc557 100644
--- a/lib/iris/tests/experimental/test_animate.py
+++ b/lib/iris/tests/experimental/test_animate.py
@@ -1,4 +1,4 @@
-# (C) British Crown Copyright 2013 - 2015, Met Office
+# (C) British Crown Copyright 2013 - 2016, Met Office
 #
 # This file is part of Iris.
 #
@@ -40,6 +40,7 @@
 @tests.skip_plot
 class IntegrationTest(tests.GraphicsTest):
     def setUp(self):
+        super(IntegrationTest, self).setUp()
         cube = iris.cube.Cube(np.arange(36, dtype=np.int32).reshape((3, 3, 4)))
         cs = GeogCS(6371229)
 
diff --git a/lib/iris/tests/idiff.py b/lib/iris/tests/idiff.py
index 1e63f25a3b..f2e1364d54 100755
--- a/lib/iris/tests/idiff.py
+++ b/lib/iris/tests/idiff.py
@@ -29,7 +29,6 @@
 import codecs
 import contextlib
 from glob import glob
-import hashlib
 import json
 import os.path
 import shutil
@@ -38,10 +37,12 @@
 
 from PIL import Image
 import filelock
+import imagehash
 import matplotlib.pyplot as plt
 import matplotlib.image as mimg
 import matplotlib.testing.compare as mcompare
 import matplotlib.widgets as mwidget
+import numpy as np
 import requests
 
 # Force iris.tests to use the ```tkagg``` backend by using the '-d'
@@ -55,8 +56,6 @@
 _POSTFIX_DIFF = '-failed-diff.png'
 _POSTFIX_JSON = os.path.join('results', 'imagerepo.json')
 _POSTFIX_LOCK = os.path.join('results', 'imagerepo.lock')
-_TIMEOUT = 30
-_TOL = 0
 
 
 @contextlib.contextmanager
@@ -70,9 +69,9 @@ def temp_png(suffix=''):
         os.remove(fname)
 
 
-def diff_viewer(repo, key, repo_fname,
+def diff_viewer(repo, key, repo_fname, phash, status,
                 expected_fname, result_fname, diff_fname):
-    plt.figure(figsize=(14, 12))
+    fig = plt.figure(figsize=(14, 12))
     plt.suptitle(os.path.basename(expected_fname))
     ax = plt.subplot(221)
     ax.imshow(mimg.imread(expected_fname))
@@ -81,14 +80,11 @@ def diff_viewer(repo, key, repo_fname,
     ax = plt.subplot(223, sharex=ax, sharey=ax)
     ax.imshow(mimg.imread(diff_fname))
 
-    # Determine the new image hash.png name.
-    with open(result_fname, 'rb') as fi:
-        sha1 = hashlib.sha1(fi.read())
     result_dir = os.path.dirname(result_fname)
-    fname = sha1.hexdigest() + '.png'
-    base_uri = 'https://scitools.github.io/test-images-scitools/image_files/{}'
+    fname = '{}.png'.format(phash)
+    base_uri = 'https://scitools.github.io/test-iris-imagehash/images/{}'
     uri = base_uri.format(fname)
-    hash_fname = os.path.join(result_dir, fname)
+    phash_fname = os.path.join(result_dir, fname)
 
     def accept(event):
         if uri not in repo[key]:
@@ -100,14 +96,14 @@ def accept(event):
             with open(repo_fname, 'wb') as fo:
                 json.dump(repo, codecs.getwriter('utf-8')(fo),
                           indent=4, sort_keys=True)
-            os.rename(result_fname, hash_fname)
+            os.rename(result_fname, phash_fname)
             msg = 'ACCEPTED:  {} -> {}'
             print(msg.format(os.path.basename(result_fname),
-                             os.path.basename(hash_fname)))
+                             os.path.basename(phash_fname)))
         else:
             msg = 'DUPLICATE: {} -> {} (ignored)'
             print(msg.format(os.path.basename(result_fname),
-                             os.path.basename(hash_fname)))
+                             os.path.basename(phash_fname)))
             os.remove(result_fname)
         os.remove(diff_fname)
         plt.close()
@@ -118,7 +114,7 @@ def reject(event):
         else:
             msg = 'DUPLICATE: {} -> {} (ignored)'
             print(msg.format(os.path.basename(result_fname),
-                             os.path.basename(hash_fname)))
+                             os.path.basename(phash_fname)))
         os.remove(result_fname)
         os.remove(diff_fname)
         plt.close()
@@ -137,42 +133,99 @@ def skip(event):
     breject.on_clicked(reject)
     bskip = mwidget.Button(ax_skip, 'Skip')
     bskip.on_clicked(skip)
+    plt.text(0.59, 0.15, status, transform=fig.transFigure)
     plt.show()
 
 
-def step_over_diffs(result_dir, index):
+# TBD: Push this fix to imagehash (done!)
+# See https://github.com/JohannesBuchner/imagehash/pull/31
+# Now need this imagehash/master pushed to pypi ...
+def _hex_to_hash(hexstr, hash_size=iris.tests._HASH_SIZE):
+    l = []
+    count = hash_size * (hash_size // 4)
+    if len(hexstr) != count:
+        emsg = 'Expected hex string size of {}.'
+        raise ValueError(emsg.format(count))
+    for i in range(count // 2):
+        h = hexstr[i*2:i*2+2]
+        v = int("0x" + h, 16)
+        l.append([v & 2**i > 0 for i in range(8)])
+    return imagehash.ImageHash(np.array(l))
+
+
+def _calculate_hit(uris, phash, action):
+    # Create the expected perceptual image hashes from the uris.
+    expected = [_hex_to_hash(os.path.splitext(os.path.basename(uri))[0])
+                for uri in uris]
+    # Calculate the hamming distance vector for the result hash.
+    distances = [e - phash for e in expected]
+    if action == 'first':
+        index = 0
+    elif action == 'last':
+        index = -1
+    elif action == 'similar':
+        index = np.argmin(distances)
+    elif action == 'difference':
+        index = np.argmax(distances)
+    else:
+        emsg = 'Unknown action: {!r}'
+        raise ValueError(emsg.format(action))
+    return index, distances[index]
+
+
+def step_over_diffs(result_dir, action, display=True):
     processed = False
     dname = os.path.dirname(iris.tests.__file__)
     lock = filelock.FileLock(os.path.join(dname, _POSTFIX_LOCK))
-    prog = os.path.basename(os.path.splitext(sys.argv[0])[0])
-    msg = '\n{}: Comparing result image with {} expected test image.'
-    print(msg.format(prog, 'oldest' if index == 0 else 'youngest'))
+    if action in ['first', 'last']:
+        kind = action
+    elif action in ['similar', 'different']:
+        kind = 'most {}'.format(action)
+    else:
+        emsg = 'Unknown action: {!r}'
+        raise ValueError(emsg.format(action))
+    if display:
+        msg = ('\nComparing the {!r} expected image with '
+               'the test result image.')
+        print(msg.format(kind))
 
     # Remove old image diff results.
     target = os.path.join(result_dir, '*{}'.format(_POSTFIX_DIFF))
     for fname in glob(target):
         os.remove(fname)
 
-    with lock.acquire(timeout=_TIMEOUT):
+    with lock.acquire(timeout=30):
         # Load the imagerepo.
         repo_fname = os.path.join(dname, _POSTFIX_JSON)
         with open(repo_fname, 'rb') as fi:
             repo = json.load(codecs.getreader('utf-8')(fi))
 
-        target = os.path.join(result_dir, 'result-*.png')
-        for result_fname in sorted(glob(target)):
+        # Filter out all non-test result image files.
+        target_glob = os.path.join(result_dir, 'result-*.png')
+        results = []
+        for fname in sorted(glob(target_glob)):
             # We only care about PNG images.
             try:
-                im = Image.open(result_fname)
+                im = Image.open(fname)
                 if im.format != 'PNG':
                     # Ignore - it's not a png image.
                     continue
             except IOError:
                 # Ignore - it's not an image.
                 continue
+            results.append(fname)
+
+        count = len(results)
+
+        for count_index, result_fname in enumerate(results):
             key = os.path.splitext('-'.join(result_fname.split('-')[1:]))[0]
             try:
-                uri = repo[key][index]
+                # Calculate the test result perceptual image hash.
+                phash = imagehash.phash(Image.open(result_fname),
+                                        hash_size=iris.tests._HASH_SIZE)
+                uris = repo[key]
+                hash_index, distance = _calculate_hit(uris, phash, action)
+                uri = uris[hash_index]
             except KeyError:
                 wmsg = 'Ignoring unregistered test result {!r}.'
                 warnings.warn(wmsg.format(key))
@@ -197,39 +250,50 @@ def step_over_diffs(result_dir, index):
                         # So copy the local file to the exected file to
                         # maintain this helpfulness.
                         shutil.copy(local_fname, expected_fname)
-                mcompare.compare_images(expected_fname, result_fname, tol=_TOL)
+                mcompare.compare_images(expected_fname, result_fname, tol=0)
                 diff_fname = os.path.splitext(result_fname)[0] + _POSTFIX_DIFF
-                diff_viewer(repo, key, repo_fname, expected_fname,
-                            result_fname, diff_fname)
-        if not processed:
-            msg = '\n{}: There are no iris test result images to process.\n'
-            print(msg.format(prog))
+                args = expected_fname, result_fname, diff_fname
+                if display:
+                    msg = ('Image {} of {}: hamming distance = {} '
+                           '[{!r}]')
+                    status = msg.format(count_index+1, count, distance, kind)
+                    prefix = repo, key, repo_fname, phash, status
+                    yield prefix + args
+                else:
+                    yield args
+        if display and not processed:
+            print('\nThere are no iris test result images to process.\n')
 
 
 if __name__ == '__main__':
     default = os.path.join(os.path.dirname(iris.tests.__file__),
                            'result_image_comparison')
     description = 'Iris graphic test difference tool.'
-    parser = argparse.ArgumentParser(description=description)
-    help = 'Path to iris tests result image directory (default: %(default)s)'
+    formatter_class = argparse.RawTextHelpFormatter
+    parser = argparse.ArgumentParser(description=description,
+                                     formatter_class=formatter_class)
+    help = 'path to iris tests result image directory (default: %(default)s)'
     parser.add_argument('--resultdir', '-r',
                         default=default,
                         help=help)
-    help = 'Force "iris.tests" to use the tkagg backend (default: %(default)s)'
+    help = 'force "iris.tests" to use the tkagg backend (default: %(default)s)'
     parser.add_argument('-d',
                         action='store_true',
                         default=True,
                         help=help)
-    help = ('Compare result image with the oldest or last registered '
-            'expected test image')
-    parser.add_argument('--last', '-l',
-                        action='store_true',
-                        default=False,
-                        help=help)
+    help = """
+first     - compare result image with first (oldest) expected image
+last      - compare result image with last (youngest) expected image
+similar   - compare result image with most similar expected image (default)
+different - compare result image with most unsimilar expected image
+"""
+    choices = ('first', 'last', 'similar', 'different')
+    parser.add_argument('action', nargs='?', choices=choices,
+                        default='similar', help=help)
     args = parser.parse_args()
     result_dir = args.resultdir
     if not os.path.isdir(result_dir):
         emsg = 'Invalid results directory: {}'
         raise ValueError(emsg.format(result_dir))
-    index = -1 if args.last else 0
-    step_over_diffs(result_dir, index)
+    for args in step_over_diffs(result_dir, args.action):
+        diff_viewer(*args)
diff --git a/lib/iris/tests/integration/plot/test_colorbar.py b/lib/iris/tests/integration/plot/test_colorbar.py
index aae46b8efb..5e59bb4330 100644
--- a/lib/iris/tests/integration/plot/test_colorbar.py
+++ b/lib/iris/tests/integration/plot/test_colorbar.py
@@ -1,4 +1,4 @@
-# (C) British Crown Copyright 2014 - 2015, Met Office
+# (C) British Crown Copyright 2014 - 2016, Met Office
 #
 # This file is part of Iris.
 #
@@ -42,6 +42,7 @@
 @tests.skip_plot
 class TestColorBarCreation(tests.GraphicsTest):
     def setUp(self):
+        super(TestColorBarCreation, self).setUp()
         self.draw_functions = (contour, contourf, pcolormesh, pcolor)
         self.cube = iris.tests.stock.lat_lon_cube()
         self.cube.coord('longitude').guess_bounds()
diff --git a/lib/iris/tests/results/imagerepo.json b/lib/iris/tests/results/imagerepo.json
index 110adf8d75..fd63d6c507 100644
--- a/lib/iris/tests/results/imagerepo.json
+++ b/lib/iris/tests/results/imagerepo.json
@@ -1,921 +1,712 @@
 {
     "example_tests.test_COP_1d_plot.TestCOP1DPlot.test_COP_1d_plot.0": [
-        "https://scitools.github.io/test-images-scitools/image_files/775b9dec3549e46f9671f79899f09f5c42870b0c.png", 
-        "https://scitools.github.io/test-images-scitools/image_files/6d726f53e6e2e9aa1d812312aac27276a34616cf.png", 
-        "https://scitools.github.io/test-images-scitools/image_files/33271b3159960ea7eeff11d42d7e3f9bd5575cc2.png"
+        "https://scitools.github.io/test-iris-imagehash/images/5dff1a996c06b47193eecc52275b936d58b6239ba94c133643897c6c2333f086.png"
     ], 
     "example_tests.test_COP_maps.TestCOPMaps.test_cop_maps.0": [
-        "https://scitools.github.io/test-images-scitools/image_files/4da9442a24056728296b6637f062f42a9cd6ee06.png", 
-        "https://scitools.github.io/test-images-scitools/image_files/14505f4a4c2fdb306751de8bacfac6b44dadee85.png", 
-        "https://scitools.github.io/test-images-scitools/image_files/20d1518ef92f2187038385ca28f474d3e764bc52.png"
+        "https://scitools.github.io/test-iris-imagehash/images/57891cdba966a124897c568316997ea1a97e897ee1c6699d5681ad733c9296ac.png"
     ], 
     "example_tests.test_SOI_filtering.TestSOIFiltering.test_soi_filtering.0": [
-        "https://scitools.github.io/test-images-scitools/image_files/c998a36d81d38745ab2513af5450a39494edb2af.png", 
-        "https://scitools.github.io/test-images-scitools/image_files/73ae763ac65b57c7cfc797c04a34afd7e32cd123.png", 
-        "https://scitools.github.io/test-images-scitools/image_files/ffd8d3a3a85a12ff2b90c310c22d8aacfa62f2e5.png"
+        "https://scitools.github.io/test-iris-imagehash/images/5f23069d83de1e4e7ca0a595a9725b0f46cce499a97239a563dc594a4b725aa9.png", 
+        "https://scitools.github.io/test-iris-imagehash/images/5f21069d83de1e4e7ca0a595a9725b0f46ccec99a97239a563dc594a4b7252b9.png"
     ], 
     "example_tests.test_TEC.TestTEC.test_TEC.0": [
-        "https://scitools.github.io/test-images-scitools/image_files/463935ede08ceddadcc24324e0a11c24fb46a0c8.png", 
-        "https://scitools.github.io/test-images-scitools/image_files/ef9eaafe15d78b0dca80674e2b4183d06d957af1.png"
+        "https://scitools.github.io/test-iris-imagehash/images/87a5866dd958594221765992e39a763c733609de5cc1837ed8419ccd27cfdc23.png", 
+        "https://scitools.github.io/test-iris-imagehash/images/87a5866dd95879c221765992e39a7634733609de5cc18376d8498ccd27cfdc31.png"
     ], 
     "example_tests.test_anomaly_log_colouring.TestAnomalyLogColouring.test_anomaly_log_colouring.0": [
-        "https://scitools.github.io/test-images-scitools/image_files/19db1152a859567b0bf9fe89a7d1880acafbf100.png", 
-        "https://scitools.github.io/test-images-scitools/image_files/6e2454e989a05be593bd1082f905a9fd46c2049a.png", 
-        "https://scitools.github.io/test-images-scitools/image_files/0ee76c4de7f5e3c47198f191ab825c8974874dc7.png"
+        "https://scitools.github.io/test-iris-imagehash/images/37222687a1c5f9c9c978d9788996b69492bb67677c64255e5acb89c9b1595a30.png"
     ], 
     "example_tests.test_atlantic_profiles.TestAtlanticProfiles.test_atlantic_profiles.0": [
-        "https://scitools.github.io/test-images-scitools/image_files/0bde47fbbc4a15498c0ecb8bd3eb5e63339e5121.png", 
-        "https://scitools.github.io/test-images-scitools/image_files/b8365851d0b4b5589289026eda692c643499c7f3.png", 
-        "https://scitools.github.io/test-images-scitools/image_files/c3a2ccc6873ddee7ea0dafd25498b743ffece4fa.png", 
-        "https://scitools.github.io/test-images-scitools/image_files/4cbda27eb65fc360d08dde0ccf40b951fdfe4453.png", 
-        "https://scitools.github.io/test-images-scitools/image_files/3a2f43bc6b1bd4397a8eb6877a128ab7a4f051ff.png", 
-        "https://scitools.github.io/test-images-scitools/image_files/da5cd8269bcc96533c8c2f4a788e2c980381dfef.png"
+        "https://scitools.github.io/test-iris-imagehash/images/f94106cad64b71c804ce29ecad2fec0da5b8662f33ba103f0bf0db38c93f4d38.png", 
+        "https://scitools.github.io/test-iris-imagehash/images/f94106cad64b71c804ce29ecad2fec0da5b8662f333e902f0bf0db38c93f4d38.png"
     ], 
     "example_tests.test_atlantic_profiles.TestAtlanticProfiles.test_atlantic_profiles.1": [
-        "https://scitools.github.io/test-images-scitools/image_files/ddc7f58664e61c5798ecfc7e55ec3c6c62ab1a24.png", 
-        "https://scitools.github.io/test-images-scitools/image_files/b26878c2fabfdc727787dd98f465043bbbfd3553.png"
+        "https://scitools.github.io/test-iris-imagehash/images/6557a57e7681bb9f998cd8c5cdee7a0421ba1a918399e6dc724425e67a318538.png"
     ], 
     "example_tests.test_coriolis_plot.TestCoriolisPlot.test_coriolis_plot.0": [
-        "https://scitools.github.io/test-images-scitools/image_files/7576971ae4fce5456c67c5d54f3f4110d6e85e13.png", 
-        "https://scitools.github.io/test-images-scitools/image_files/fb246b208d11834e8108067504c6a73ee05f85ee.png", 
-        "https://scitools.github.io/test-images-scitools/image_files/e675b4d1cc5d38da66de49a1f204bc1cc983ebba.png"
+        "https://scitools.github.io/test-iris-imagehash/images/e761a67b5996699aa77a99a611969e699661a3677c1983795ca4669e87195824.png"
     ], 
     "example_tests.test_cross_section.TestCrossSection.test_cross_section.0": [
-        "https://scitools.github.io/test-images-scitools/image_files/7b5c6d7466e953e845d0bc1ec9f08b01471b87ee.png", 
-        "https://scitools.github.io/test-images-scitools/image_files/a4e6e12b25cacfcb7c112bec3cec754c3b165a60.png", 
-        "https://scitools.github.io/test-images-scitools/image_files/a09ea89f76efcdbfe0a8e130bf9c991961150794.png"
+        "https://scitools.github.io/test-iris-imagehash/images/57a98cdea946278b26f95aa0a17632255b4a297e72a58ffcd892b9428fdcd882.png"
     ], 
     "example_tests.test_cross_section.TestCrossSection.test_cross_section.1": [
-        "https://scitools.github.io/test-images-scitools/image_files/3589f5e242ce5b565b74da23287fddec32d102fa.png", 
-        "https://scitools.github.io/test-images-scitools/image_files/4ddf3595a4c21da1ea984d2bd377ff45c1ffd37b.png"
+        "https://scitools.github.io/test-iris-imagehash/images/57a984dfa9569c02964978c90ffe52b5a136237e72a9a15e78a55bac895dd881.png"
     ], 
     "example_tests.test_custom_aggregation.TestCustomAggregation.test_custom_aggregation.0": [
-        "https://scitools.github.io/test-images-scitools/image_files/3fd959e6f86fbe99ff0fa9fb06d5af48edde69a6.png", 
-        "https://scitools.github.io/test-images-scitools/image_files/53e6e605d77d92b7a565a05b94fbe71b0884dcfe.png", 
-        "https://scitools.github.io/test-images-scitools/image_files/33d66426e41b24bfc1135e2c03390aa73cac44c0.png"
+        "https://scitools.github.io/test-iris-imagehash/images/7f817681897e097e2d7ce1fca1e65e83090f0e3c56a981f858c33c87a55ef618.png"
     ], 
     "example_tests.test_custom_file_loading.TestCustomFileLoading.test_custom_file_loading.0": [
-        "https://scitools.github.io/test-images-scitools/image_files/c903e396c04a4d1076767c8474b1e48c80413aab.png", 
-        "https://scitools.github.io/test-images-scitools/image_files/73399ce0ab66b9237ea3573390a6abc95a86393e.png"
+        "https://scitools.github.io/test-iris-imagehash/images/5f05d38f217a2c7d89ece182763b133ddc13f8d9c6cc644625b70cb3834db384.png", 
+        "https://scitools.github.io/test-iris-imagehash/images/df05d38f217a2c7d89e4e182763b133ddc11f8d946cce446a5b54cb3834db384.png"
     ], 
     "example_tests.test_deriving_phenomena.TestDerivingPhenomena.test_deriving_phenomena.0": [
-        "https://scitools.github.io/test-images-scitools/image_files/4f6ee4c1ad1b3fa3c5d31ddeba34c8ec544fed50.png", 
-        "https://scitools.github.io/test-images-scitools/image_files/c127277d79e408af8d9826269858168632629e83.png"
+        "https://scitools.github.io/test-iris-imagehash/images/9d999c6161964a679362629c236ed69b63968976de998366fc996e91096e7c81.png"
     ], 
     "example_tests.test_global_map.TestGlobalMap.test_global_map.0": [
-        "https://scitools.github.io/test-images-scitools/image_files/c4193550f97b1384419c0761ba85304012638ce8.png", 
-        "https://scitools.github.io/test-images-scitools/image_files/0d370726237672639b01eda6795297af1e7153d3.png", 
-        "https://scitools.github.io/test-images-scitools/image_files/2f0db2900216842956d316b3a92831f3bcff8a72.png"
+        "https://scitools.github.io/test-iris-imagehash/images/5f999e62a166a17e0f7e7c911e6ad689d3809e113c9129666195d6b9c16ef681.png"
     ], 
     "example_tests.test_hovmoller.TestGlobalMap.test_hovmoller.0": [
-        "https://scitools.github.io/test-images-scitools/image_files/6de52bb3dc3838802c87fedfcdd3db585bae4ccf.png", 
-        "https://scitools.github.io/test-images-scitools/image_files/12ee81f8bd07182dd5165a0e77c4cf3b746fa93a.png"
+        "https://scitools.github.io/test-iris-imagehash/images/5d2d0c2d73d273c2a37df391a3d258c6a3c2a3767826097edc2d962d097b5883.png"
     ], 
     "example_tests.test_inset_plot.TestInsetPlot.test_inset_plot.0": [
-        "https://scitools.github.io/test-images-scitools/image_files/a4d290bab6da123ef455b39979f415debb2b925c.png", 
-        "https://scitools.github.io/test-images-scitools/image_files/414dadf1e840ec67c96612e4ac832019bd605bd5.png", 
-        "https://scitools.github.io/test-images-scitools/image_files/988896162aa71b6cb7034dae4a221feacd38094c.png"
+        "https://scitools.github.io/test-iris-imagehash/images/d7ff9649af0069a54da25b236faa29692d4912db93bad396a99489b4f324522a.png", 
+        "https://scitools.github.io/test-iris-imagehash/images/97ff9649ad0069a54da25b236fa2296d2d4912db93bad396a994a9b4f324526a.png"
     ], 
     "example_tests.test_lagged_ensemble.TestLaggedEnsemble.test_lagged_ensemble.0": [
-        "https://scitools.github.io/test-images-scitools/image_files/c23632eba452a7c3074396c288ddc85cf9a27923.png", 
-        "https://scitools.github.io/test-images-scitools/image_files/ed98c2a750eae4f06b269c38ca3810ce9eb51c38.png", 
-        "https://scitools.github.io/test-images-scitools/image_files/8f68ed33e818f4cde38bc94c755e2a98b58016fe.png", 
-        "https://scitools.github.io/test-images-scitools/image_files/6624fb203d0d905c5dc49f362ce502932692ed3f.png"
+        "https://scitools.github.io/test-iris-imagehash/images/dddd8c87237226270da2d9da8d8e765323262f69732c86718de0d99c8dc973a4.png"
     ], 
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-        "https://scitools.github.io/test-images-scitools/image_files/2aae63c8a7581a0a84c3ec3df73cf96a3afa6cda.png", 
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-        "https://scitools.github.io/test-images-scitools/image_files/086e2955203872dcdbdeef91b335f618231cfb18.png"
+        "https://scitools.github.io/test-iris-imagehash/images/d57f9f1abf6234995ce01b9e0662d2818b0069e1839ccbad2997f3e1631d69e1.png"
     ], 
     "example_tests.test_lineplot_with_legend.TestLineplotWithLegend.test_lineplot_with_legend.0": [
-        "https://scitools.github.io/test-images-scitools/image_files/f4d65c3ff5b1c11617479af2977279f3b9c743bd.png", 
-        "https://scitools.github.io/test-images-scitools/image_files/b0483b5e302db34c4d6ca74a729cc6c9056e2b36.png", 
-        "https://scitools.github.io/test-images-scitools/image_files/50fd225845d2dec87e56e63534564d5727dc60cc.png"
+        "https://scitools.github.io/test-iris-imagehash/images/5797424aa6021d9669f8b165291ab965a9c233598f8052dfc3bf9ad6217f98e5.png"
     ], 
     "example_tests.test_orca_projection.TestOrcaProjection.test_orca_projection.0": [
-        "https://scitools.github.io/test-images-scitools/image_files/31770e35ff8b5d35be3d167362ad3beed4ab9bde.png", 
-        "https://scitools.github.io/test-images-scitools/image_files/fdb6621e8a1a84845411329e86e773e5a40ffbb4.png"
+        "https://scitools.github.io/test-iris-imagehash/images/df88ce582973257726cd7a898b4b0c72795ae39cde04877f48a124e167667362.png"
     ], 
     "example_tests.test_orca_projection.TestOrcaProjection.test_orca_projection.1": [
-        "https://scitools.github.io/test-images-scitools/image_files/12e559666b9116918c861fe09ddcf989f35699d4.png", 
-        "https://scitools.github.io/test-images-scitools/image_files/38cb106e87d8540d9999bd47587e718244ca398d.png"
+        "https://scitools.github.io/test-iris-imagehash/images/a765a665599a699aa7db18a643a6dc61996933677c9987595889649ce71878a6.png"
     ], 
     "example_tests.test_orca_projection.TestOrcaProjection.test_orca_projection.2": [
-        "https://scitools.github.io/test-images-scitools/image_files/eef5b0eb8018eb07bbf9457c0641d062baa289a7.png", 
-        "https://scitools.github.io/test-images-scitools/image_files/b97f483476f3006f83886fd6746a38193b65a16f.png", 
-        "https://scitools.github.io/test-images-scitools/image_files/ddbcb3a5f795c9e064ea6b42bbb06fe65c80673e.png", 
-        "https://scitools.github.io/test-images-scitools/image_files/4f5a4c1ed5e0a3bedeac0433212c8b2296af68b4.png"
+        "https://scitools.github.io/test-iris-imagehash/images/4f232673799cc94c87ed3287339ecc31a661a7cddc8ccd5e6693663203765826.png"
     ], 
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-        "https://scitools.github.io/test-images-scitools/image_files/76a40975b3e35f8226c37be146ad6fc678eea13a.png", 
-        "https://scitools.github.io/test-images-scitools/image_files/81c58c8883d143a774546e8c368e5468a9f691cc.png"
+        "https://scitools.github.io/test-iris-imagehash/images/5f815ec121762536a79c93cc897b4c3361f3e1c5fc85164e38db7c9176ecd220.png"
     ], 
     "example_tests.test_polar_stereo.TestPolarStereo.test_polar_stereo.0": [
-        "https://scitools.github.io/test-images-scitools/image_files/889b91c7a5ff2403e24dce5047e95dc7eec7efe7.png", 
-        "https://scitools.github.io/test-images-scitools/image_files/9dc8f0fb0201cca91dd00f0ede59c326f0d8a8f4.png", 
-        "https://scitools.github.io/test-images-scitools/image_files/daa99a98023de225e84ff8230cd5f0e60c451b30.png"
+        "https://scitools.github.io/test-iris-imagehash/images/87168c5e49cbb691a3dd7929a37af63059c9835a76a3216edc848ee6897b5c81.png"
     ], 
     "example_tests.test_polynomial_fit.TestPolynomialFit.test_polynomial_fit.0": [
-        "https://scitools.github.io/test-images-scitools/image_files/63876535ed13433a906cd08480032b741194e326.png", 
-        "https://scitools.github.io/test-images-scitools/image_files/be0c6a43152e1b3b5e16de751423d93ccdfe6c6e.png", 
-        "https://scitools.github.io/test-images-scitools/image_files/c7390b915798d1c3c793d71f22cb0e2d3524d294.png"
+        "https://scitools.github.io/test-iris-imagehash/images/d5ff52b94f26ac11a60493fe488262a9236db9c4c9d213563b6949ec6b3133f6.png", 
+        "https://scitools.github.io/test-iris-imagehash/images/55ff52b94f26ac11a60493fe488262a9236db9c4c9d213563b6959e86b3933f6.png"
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+        "https://scitools.github.io/test-iris-imagehash/images/175fb5e2ef21bda069a1b9c069f9994603ba9376078e917f160e1346168e1e1f.png", 
+        "https://scitools.github.io/test-iris-imagehash/images/155fb4e2e9a1a9a16da9b9e006fa9176078a965e0b869b5f0b8659751b807b75.png"
     ], 
     "iris.tests.test_quickplot.TestQuickplotCoordinatesGiven.test_x.0": [
-        "https://scitools.github.io/test-images-scitools/image_files/8ab60a9149fe130e8ee33dafce5a291f832465c1.png", 
-        "https://scitools.github.io/test-images-scitools/image_files/177ec14024ed9ad1e201e307cd5669eefd9c25f1.png", 
-        "https://scitools.github.io/test-images-scitools/image_files/baac265d11d068d939071f728e9876fb2181e487.png"
+        "https://scitools.github.io/test-iris-imagehash/images/65ffad907e21b34744a2198b26f9b1364b1c316e0b9e19e60b905b6e931f1b66.png", 
+        "https://scitools.github.io/test-iris-imagehash/images/65fdad90fe21b34744a2998b26f9b1364b1c316e0b9e19e60b905b6e831f1b66.png"
     ], 
     "iris.tests.test_quickplot.TestQuickplotCoordinatesGiven.test_y.0": [
-        "https://scitools.github.io/test-images-scitools/image_files/8b803be114227b39cb4bf41fe8dd1355b31ca83b.png", 
-        "https://scitools.github.io/test-images-scitools/image_files/b3f073dfedf52dfec9814c97211afb67678af2cb.png", 
-        "https://scitools.github.io/test-images-scitools/image_files/8c133bbce67f944b1ca505577c2acbbeea4fb7de.png"
+        "https://scitools.github.io/test-iris-imagehash/images/e5ffe9d1fe0093130ceb998466e87969a9901b66231e9b260b9e136e1b965b64.png"
     ], 
     "iris.tests.test_quickplot.TestQuickplotCoordinatesGiven.test_yx.0": [
-        "https://scitools.github.io/test-images-scitools/image_files/58980b9a539d71094318403999772cdc7ef480b1.png", 
-        "https://scitools.github.io/test-images-scitools/image_files/1fc87fa03b788ff7390ec218551e6fae91155e70.png"
+        "https://scitools.github.io/test-iris-imagehash/images/57a156f98f76ed02a95279a0a15a98c503df837c78a503be5a0bd31a277a3cac.png"
     ], 
     "iris.tests.test_quickplot.TestQuickplotCoordinatesGiven.test_yx.1": [
-        "https://scitools.github.io/test-images-scitools/image_files/0650e2de7082c3e2052758c584854023adce4c07.png", 
-        "https://scitools.github.io/test-images-scitools/image_files/76b9ef820329b58d2a4a0ec8bddc2cde3a9b6dcf.png"
+        "https://scitools.github.io/test-iris-imagehash/images/a7a5a66d79585942897e589883de5c86799a23de5ca4a37cdc210ca7a35e7ca1.png"
     ], 
     "iris.tests.test_quickplot.TestQuickplotCoordinatesGiven.test_yx.2": [
-        "https://scitools.github.io/test-images-scitools/image_files/a6c192b994f0781b3538a794bdddff0d84bd9954.png", 
-        "https://scitools.github.io/test-images-scitools/image_files/3759871c9cb796985b09699de633f29f2f054d01.png", 
-        "https://scitools.github.io/test-images-scitools/image_files/ad13f00d1d88ba2c903def2fffbdf0d932cbaced.png"
+        "https://scitools.github.io/test-iris-imagehash/images/f5ff676bee00a9616ce8b949079899b40b9c5baf81be195e015e1227991652b6.png", 
+        "https://scitools.github.io/test-iris-imagehash/images/75ff676bee01a9616ce8b949079891b48b9c5baf81ba195e015e1267991652b6.png"
     ], 
     "iris.tests.test_quickplot.TestQuickplotCoordinatesGiven.test_yx.3": [
-        "https://scitools.github.io/test-images-scitools/image_files/71ba9935c4c4d7a5de8c94318b1b886ce9ee27cb.png", 
-        "https://scitools.github.io/test-images-scitools/image_files/6b37a0e9e7ce43b585828f57a9006f9f9dee4d0e.png"
+        "https://scitools.github.io/test-iris-imagehash/images/577a86212c356ca783936cd9a9cd391cc3c559f273875976d926d30699a4b3a4.png"
     ], 
     "iris.tests.test_quickplot.TestQuickplotCoordinatesGiven.test_yx.4": [
-        "https://scitools.github.io/test-images-scitools/image_files/139739470587376aebae270abeca5100e2e996ff.png", 
-        "https://scitools.github.io/test-images-scitools/image_files/82737f30059330b4d1b8286110a482ff9e2975f1.png", 
-        "https://scitools.github.io/test-images-scitools/image_files/1f62abf9f24fa817f4308fd86f876a3e3cf20e12.png"
+        "https://scitools.github.io/test-iris-imagehash/images/b5f4b6f44b0b49a9438bc3cb3cd8436bbe34963607a63c5c438d9b6e5ba04323.png"
     ], 
     "iris.tests.test_quickplot.TestQuickplotCoordinatesGiven.test_yx.5": [
-        "https://scitools.github.io/test-images-scitools/image_files/6e2ee19aab2fdcdf3e3cb940987abcd1d682fe48.png", 
-        "https://scitools.github.io/test-images-scitools/image_files/0bf0adf4e0c7998c6b5e879d2af1e9dec4e13359.png"
+        "https://scitools.github.io/test-iris-imagehash/images/9716a671696949e3e9e179dc6149791a96b692b3169693c59e4693c499b039b6.png", 
+        "https://scitools.github.io/test-iris-imagehash/images/9787a6716978496a79793c9e66cb361a86a696310e8773ced96c198679781932.png"
     ], 
     "iris.tests.test_quickplot.TestQuickplotCoordinatesGiven.test_zx.0": [
-        "https://scitools.github.io/test-images-scitools/image_files/e8f27272cfb56af77be12f9652a63842a0fd9ce8.png", 
-        "https://scitools.github.io/test-images-scitools/image_files/14127d16204049c9463f8ed7d5c51cd250dc7556.png"
+        "https://scitools.github.io/test-iris-imagehash/images/fd81fc01836a03ba037f43b983fe58b60bfa03ff5885a37edc24dc04ec5a7881.png"
     ], 
     "iris.tests.test_quickplot.TestQuickplotCoordinatesGiven.test_zx.1": [
-        "https://scitools.github.io/test-images-scitools/image_files/3bced65ec9fdea60baeeea8a1743f9e7ab6f876f.png", 
-        "https://scitools.github.io/test-images-scitools/image_files/cd24c4923efaedc1ef5b19ee39d729cc88ccf23f.png"
+        "https://scitools.github.io/test-iris-imagehash/images/57a946b9a956990696c979d903fe5eb5a136237e7a81a15e78a452ac837dd881.png"
     ], 
     "iris.tests.test_quickplot.TestQuickplotCoordinatesGiven.test_zx.2": [
-        "https://scitools.github.io/test-images-scitools/image_files/c83f0af560a06159f052d19b67166bf1636028e3.png", 
-        "https://scitools.github.io/test-images-scitools/image_files/a4e99fe82705b50dcc6f260779c604a3600fb2e9.png", 
-        "https://scitools.github.io/test-images-scitools/image_files/6d7842f31a67ae32c7d050e818424573f37e442e.png"
+        "https://scitools.github.io/test-iris-imagehash/images/e1b7f461fe00b14104e8194a0ff89b7d8b1e5936213e13ee2356934e197e13bf.png", 
+        "https://scitools.github.io/test-iris-imagehash/images/e1b7f460fe00b14104e8194a0ff89b7d8b1e5936213e13ee2316936f197e13bf.png", 
+        "https://scitools.github.io/test-iris-imagehash/images/e1b7f460fe00b14104e8394a0ff89b7d8b1e5936213e13eea306936e197e13bf.png"
     ], 
     "iris.tests.test_quickplot.TestQuickplotCoordinatesGiven.test_zx.3": [
-        "https://scitools.github.io/test-images-scitools/image_files/995656ea051a42c5e2c0f27e71ab23fd929ca5a7.png", 
-        "https://scitools.github.io/test-images-scitools/image_files/6a0cb7ec5858f9416a1fe0e3be5bb051cc205cfd.png", 
-        "https://scitools.github.io/test-images-scitools/image_files/87cb7c6c2507fdeb600f3ff09223d81bdc6ad670.png"
+        "https://scitools.github.io/test-iris-imagehash/images/459fad5d6e00a59646fb79690fb893642319336221feb9360b24d28f59d6988f.png", 
+        "https://scitools.github.io/test-iris-imagehash/images/459fad5d6e00a19646fb79690fb8b3642319336221feb9360b24d28f59d698ae.png"
     ], 
     "iris.tests.test_quickplot.TestQuickplotCoordinatesGiven.test_zx.4": [
-        "https://scitools.github.io/test-images-scitools/image_files/da3401dbe9a72f6d5440e503077f3c5a8c822530.png", 
-        "https://scitools.github.io/test-images-scitools/image_files/f15a6d1b89260e180d6a12cbe90c36953930e5ba.png", 
-        "https://scitools.github.io/test-images-scitools/image_files/2a963195b2d538cf3fd443c04632187a4af5e76d.png"
+        "https://scitools.github.io/test-iris-imagehash/images/75a9fce1ab17b46101f8897776a8897f7e881e6e03be16ee0b161b1e1b000b5e.png", 
+        "https://scitools.github.io/test-iris-imagehash/images/75a9fce1ab17b4e101d8897776a8997f7e881e2e03be16ee0b161b1e1b000b5e.png"
     ], 
     "iris.tests.test_quickplot.TestQuickplotCoordinatesGiven.test_zx.5": [
-        "https://scitools.github.io/test-images-scitools/image_files/d2fe7bb8377160a3e7f6fff364d7b6e95cf03bc3.png", 
-        "https://scitools.github.io/test-images-scitools/image_files/ba9a2021710f0c148ac1dff99fb1b3f117b1cfc9.png", 
-        "https://scitools.github.io/test-images-scitools/image_files/071aa7880cf6ac51743c29cab96672c86e520a17.png"
+        "https://scitools.github.io/test-iris-imagehash/images/175ea9b469a196c269f9966269a1b172078ab97607fed9561e86d9549e8e5854.png", 
+        "https://scitools.github.io/test-iris-imagehash/images/175ea9b469a196c269f9966269a1b152078ab97607fe99561e8ed9549e8e585c.png"
     ], 
     "iris.tests.test_quickplot.TestTimeReferenceUnitsLabels.test_not_reference_time_units.0": [
-        "https://scitools.github.io/test-images-scitools/image_files/632a054a5fadd90e17de14d4052107801ce99f26.png", 
-        "https://scitools.github.io/test-images-scitools/image_files/3def630430310102bd2d81a925bca02f795deb48.png", 
-        "https://scitools.github.io/test-images-scitools/image_files/e822dd65eef18122ed2f17ddfd080f8ebbeb8dd3.png"
+        "https://scitools.github.io/test-iris-imagehash/images/415f85e9fefb91e94600bb6f07009be7effa1966ab065b273b009b663b007a04.png", 
+        "https://scitools.github.io/test-iris-imagehash/images/411d85e9fefb91e14600bb6707009be7effe1966ab06fb273b009b663f007a04.png", 
+        "https://scitools.github.io/test-iris-imagehash/images/411f85e9fefb91e14600bb6f07009be7effe1966ab067b273b009b663b007a04.png"
     ], 
     "iris.tests.test_quickplot.TestTimeReferenceUnitsLabels.test_reference_time_units.0": [
-        "https://scitools.github.io/test-images-scitools/image_files/5da05ddeaaa666336d8daf1d1bc2473fda1b1fd0.png", 
-        "https://scitools.github.io/test-images-scitools/image_files/47cab4d1931d1a28b4bb59ba985b1f4a9992ec30.png"
+        "https://scitools.github.io/test-iris-imagehash/images/417f8119feebeeff070054bb2b0014a0bb157ba6bb972b46dabf3b0419827b04.png", 
+        "https://scitools.github.io/test-iris-imagehash/images/417f8119fefbeeff070054b92b0014a0bb557ba69b95ab46dabf3b0419827b04.png"
     ]
 }
\ No newline at end of file
diff --git a/lib/iris/tests/runner/_runner.py b/lib/iris/tests/runner/_runner.py
index 95f6b600d7..9d69c061ed 100644
--- a/lib/iris/tests/runner/_runner.py
+++ b/lib/iris/tests/runner/_runner.py
@@ -1,4 +1,4 @@
-# (C) British Crown Copyright 2010 - 2015, Met Office
+# (C) British Crown Copyright 2010 - 2016, Met Office
 #
 # This file is part of Iris.
 #
@@ -24,39 +24,18 @@
 
 # Because this file is imported by setup.py, there may be additional runtime
 # imports later in the file.
-import glob
 import multiprocessing
 import os
 import sys
 
 
-def _failed_images_iter():
-    """
-    Return a generator of [expected, actual, diff] filenames for all failed
-    image tests since the test output directory was created.
-    """
-    baseline_img_dir = os.path.join(os.path.dirname(__file__), os.path.pardir,
-                                    'results', 'visual_tests')
-    diff_dir = os.path.join(os.path.dirname(__file__), os.path.pardir,
-                            'result_image_comparison')
-    if not os.access(diff_dir, os.W_OK):
-        diff_dir = os.path.join(os.getcwd(), 'iris_image_test_output')
-
-    baselines = sorted(glob.glob(os.path.join(baseline_img_dir, '*.png')))
-    for expected_fname in baselines:
-        result_fname = os.path.join(
-            diff_dir,
-            'result-' + os.path.basename(expected_fname))
-        diff_fname = result_fname[:-4] + '-failed-diff.png'
-        if os.path.exists(diff_fname):
-            yield expected_fname, result_fname, diff_fname
-
-
 def failed_images_html():
     """
     Generates HTML which shows the image failures side-by-side
     when viewed in a web browser.
     """
+    from iris.tests.idiff import step_over_diffs
+
     data_uri_template = '<img alt="{alt}" src="data:image/png;base64,{img}">'
 
     def image_as_base64(fname):
@@ -64,8 +43,12 @@ def image_as_base64(fname):
             return fh.read().encode("base64").replace("\n", "")
 
     html = ['<!DOCTYPE html>', '<html>', '<body>']
+    rdir = os.path.join(os.path.dirname(__file__), os.path.pardir,
+                        'result_image_comparison')
+    if not os.access(rdir, os.W_OK):
+        rdir = os.path.join(os.getcwd(), 'iris_image_test_output')
 
-    for expected, actual, diff in _failed_images_iter():
+    for expected, actual, diff in step_over_diffs(rdir, 'similar', False):
         expected_html = data_uri_template.format(
             alt='expected', img=image_as_base64(expected))
         actual_html = data_uri_template.format(
diff --git a/lib/iris/tests/test_coordsystem.py b/lib/iris/tests/test_coordsystem.py
index 37108890fc..0c1f7a501b 100644
--- a/lib/iris/tests/test_coordsystem.py
+++ b/lib/iris/tests/test_coordsystem.py
@@ -382,7 +382,6 @@ def test_as_cartopy_projection(self):
         self.assertEqual(res, expected)
 
 class Test_LambertConformal(tests.GraphicsTest):
-
     def test_north_cutoff(self):
         lcc = LambertConformal(0, 0, secant_latitudes=(30, 60))
         ccrs = lcc.as_cartopy_crs()
diff --git a/lib/iris/tests/test_hybrid.py b/lib/iris/tests/test_hybrid.py
index e474d97e22..eeef1c394d 100644
--- a/lib/iris/tests/test_hybrid.py
+++ b/lib/iris/tests/test_hybrid.py
@@ -1,4 +1,4 @@
-# (C) British Crown Copyright 2010 - 2015, Met Office
+# (C) British Crown Copyright 2010 - 2016, Met Office
 #
 # This file is part of Iris.
 #
@@ -39,6 +39,7 @@
 @tests.skip_plot
 class TestRealistic4d(tests.GraphicsTest):
     def setUp(self):
+        super(TestRealistic4d, self).setUp()
         self.cube = iris.tests.stock.realistic_4d()
         self.altitude = self.cube.coord('altitude')
 
diff --git a/lib/iris/tests/test_image_json.py b/lib/iris/tests/test_image_json.py
index ae144ce3f6..9a26006369 100644
--- a/lib/iris/tests/test_image_json.py
+++ b/lib/iris/tests/test_image_json.py
@@ -34,10 +34,11 @@
 from six.moves.queue import Queue
 from threading import Thread
 
-# maximum number of threads for multi-threading code
+
+# Maximum number of threads for multi-threading code.
 MAXTHREADS = 8
 
-# Turn down requests logging
+# Turn down requests logging.
 logging.getLogger("requests").setLevel(logging.CRITICAL)
 
 
@@ -45,6 +46,7 @@ class _ResolveWorkerThread(Thread):
     """
     A :class:threading.Thread which moves objects from an input queue to an
     output deque using a 'dowork' method, as defined by a subclass.
+
     """
     def __init__(self, aqueue, adeque, exceptions):
         self.queue = aqueue
@@ -78,8 +80,9 @@ def test_resolve(self):
         uri_list = deque()
         exceptions = deque()
         uri_queue = Queue()
+        prefix = 'https://scitools.github.io/test-iris-imagehash'
         for uri in uris:
-            if uri.startswith('https://scitools.github.io'):
+            if uri.startswith(prefix):
                 uri_queue.put(uri)
             else:
                 msg = '{} is not a valid resource.'.format(uri)
diff --git a/lib/iris/tests/test_mapping.py b/lib/iris/tests/test_mapping.py
index b2ff9062fc..665a3097f4 100644
--- a/lib/iris/tests/test_mapping.py
+++ b/lib/iris/tests/test_mapping.py
@@ -1,4 +1,4 @@
-# (C) British Crown Copyright 2010 - 2015, Met Office
+# (C) British Crown Copyright 2010 - 2016, Met Office
 #
 # This file is part of Iris.
 #
@@ -81,6 +81,7 @@ def test_default_projection_and_extent(self):
 @tests.skip_plot
 class TestUnmappable(tests.GraphicsTest):
     def setUp(self):
+        super(TestUnmappable, self).setUp()
         src_cube = iris.tests.stock.global_pp()
 
         # Make a cube that can't be located on the globe.
@@ -107,6 +108,7 @@ def test_simple(self):
 @tests.skip_plot
 class TestMappingSubRegion(tests.GraphicsTest):
     def setUp(self):
+        super(TestMappingSubRegion, self).setUp()
         cube_path = tests.get_data_path(
             ('PP', 'aPProt1', 'rotatedMHtimecube.pp'))
         cube = iris.load_cube(cube_path)[0]
@@ -155,6 +157,7 @@ def test_default_projection_and_extent(self):
 @tests.skip_plot
 class TestLowLevel(tests.GraphicsTest):
     def setUp(self):
+        super(TestLowLevel, self).setUp()
         self.cube = iris.tests.stock.global_pp()
         self.few = 4
         self.few_levels = list(range(280, 300, 5))
@@ -187,6 +190,7 @@ def test_keywords(self):
 @tests.skip_plot
 class TestBoundedCube(tests.GraphicsTest):
     def setUp(self):
+        super(TestBoundedCube, self).setUp()
         self.cube = iris.tests.stock.global_pp()
         # Add some bounds to this data (this will actually make the bounds
         # invalid as they will straddle the north pole and overlap on the
@@ -221,6 +225,7 @@ def test_default_projection_and_extent(self):
 @tests.skip_plot
 class TestLimitedAreaCube(tests.GraphicsTest):
     def setUp(self):
+        super(TestLimitedAreaCube, self).setUp()
         cube_path = tests.get_data_path(('PP', 'aPProt1', 'rotated.pp'))
         self.cube = iris.load_cube(cube_path)[::20, ::20]
         self.cube.coord('grid_latitude').guess_bounds()
diff --git a/lib/iris/tests/test_plot.py b/lib/iris/tests/test_plot.py
index 026fc59acc..63b50eef93 100644
--- a/lib/iris/tests/test_plot.py
+++ b/lib/iris/tests/test_plot.py
@@ -1,4 +1,4 @@
-# (C) British Crown Copyright 2010 - 2015, Met Office
+# (C) British Crown Copyright 2010 - 2016, Met Office
 #
 # This file is part of Iris.
 #
@@ -104,6 +104,7 @@ def test_missing_cs(self):
 @tests.skip_plot
 class TestHybridHeight(tests.GraphicsTest):
     def setUp(self):
+        super(TestHybridHeight, self).setUp()
         self.cube = iris.tests.stock.realistic_4d()[0, :15, 0, :]
 
     def _check(self, plt_method, test_altitude=True):
@@ -154,6 +155,7 @@ class Test1dPlotMultiArgs(tests.GraphicsTest):
     # tests for iris.plot using multi-argument calling convention
 
     def setUp(self):
+        super(Test1dPlotMultiArgs, self).setUp()
         self.cube1d = _load_4d_testcube()[0, :, 0, 0]
         self.draw_method = iplt.plot
 
@@ -239,6 +241,7 @@ class Test1dQuickplotPlotMultiArgs(Test1dPlotMultiArgs):
     # tests for iris.plot using multi-argument calling convention
 
     def setUp(self):
+        tests.GraphicsTest.setUp(self)
         self.cube1d = _load_4d_testcube()[0, :, 0, 0]
         self.draw_method = qplt.plot
 
@@ -246,8 +249,8 @@ def setUp(self):
 @tests.skip_data
 @tests.skip_plot
 class Test1dScatter(tests.GraphicsTest):
-
     def setUp(self):
+        super(Test1dScatter, self).setUp()
         self.cube = iris.load_cube(
             tests.get_data_path(('NAME', 'NAMEIII_trajectory.txt')),
             'Temperature')
@@ -316,8 +319,8 @@ def test_not_cube_or_coord(self):
 @tests.skip_data
 @tests.skip_plot
 class Test1dQuickplotScatter(Test1dScatter):
-
     def setUp(self):
+        tests.GraphicsTest.setUp(self)
         self.cube = iris.load_cube(
             tests.get_data_path(('NAME', 'NAMEIII_trajectory.txt')),
             'Temperature')
@@ -464,6 +467,7 @@ def test_tz(self):
 class TestContour(tests.GraphicsTest, SliceMixin):
     """Test the iris.plot.contour routine."""
     def setUp(self):
+        super(TestContour, self).setUp()
         self.wind = _load_4d_testcube()
         self.draw_method = iplt.contour
 
@@ -472,6 +476,7 @@ def setUp(self):
 class TestContourf(tests.GraphicsTest, SliceMixin):
     """Test the iris.plot.contourf routine."""
     def setUp(self):
+        super(TestContourf, self).setUp()
         self.wind = _load_4d_testcube()
         self.draw_method = iplt.contourf
 
@@ -480,6 +485,7 @@ def setUp(self):
 class TestPcolor(tests.GraphicsTest, SliceMixin):
     """Test the iris.plot.pcolor routine."""
     def setUp(self):
+        super(TestPcolor, self).setUp()
         self.wind = _load_4d_testcube()
         self.draw_method = iplt.pcolor
 
@@ -488,6 +494,7 @@ def setUp(self):
 class TestPcolormesh(tests.GraphicsTest, SliceMixin):
     """Test the iris.plot.pcolormesh routine."""
     def setUp(self):
+        super(TestPcolormesh, self).setUp()
         self.wind = _load_4d_testcube()
         self.draw_method = iplt.pcolormesh
 
@@ -577,8 +584,8 @@ class TestPcolorNoBounds(six.with_metaclass(CheckForWarningsMetaclass,
     that have no bounds.
 
     """
-
     def setUp(self):
+        super(TestPcolorNoBounds, self).setUp()
         self.wind = _load_wind_no_bounds()
         self.draw_method = iplt.pcolor
 
@@ -592,8 +599,8 @@ class TestPcolormeshNoBounds(six.with_metaclass(CheckForWarningsMetaclass,
     that have no bounds.
 
     """
-
     def setUp(self):
+        super(TestPcolormeshNoBounds, self).setUp()
         self.wind = _load_wind_no_bounds()
         self.draw_method = iplt.pcolormesh
 
@@ -638,6 +645,7 @@ def test_t_dates(self):
 class TestPlot(tests.GraphicsTest, Slice1dMixin):
     """Test the iris.plot.plot routine."""
     def setUp(self):
+        super(TestPlot, self).setUp()
         self.wind = _load_4d_testcube()
         self.draw_method = iplt.plot
 
@@ -646,6 +654,7 @@ def setUp(self):
 class TestQuickplotPlot(tests.GraphicsTest, Slice1dMixin):
     """Test the iris.quickplot.plot routine."""
     def setUp(self):
+        super(TestQuickplotPlot, self).setUp()
         self.wind = _load_4d_testcube()
         self.draw_method = qplt.plot
 
@@ -687,6 +696,7 @@ def __repr__(self):
 @tests.skip_plot
 class TestPlotCoordinatesGiven(tests.GraphicsTest):
     def setUp(self):
+        super(TestPlotCoordinatesGiven, self).setUp()
         filename = tests.get_data_path(('PP', 'COLPEX',
                                         'theta_and_orog_subset.pp'))
         self.cube = load_cube_once(filename, 'air_potential_temperature')
@@ -816,6 +826,7 @@ def test_non_cube_coordinate(self):
 @tests.skip_plot
 class TestPlotDimAndAuxCoordsKwarg(tests.GraphicsTest):
     def setUp(self):
+        super(TestPlotDimAndAuxCoordsKwarg, self).setUp()
         filename = tests.get_data_path(('NetCDF', 'rotated', 'xy',
                                         'rotPole_landAreaFraction.nc'))
         self.cube = iris.load_cube(filename)
diff --git a/lib/iris/tests/test_quickplot.py b/lib/iris/tests/test_quickplot.py
index ef26e8b609..f71e7fefce 100644
--- a/lib/iris/tests/test_quickplot.py
+++ b/lib/iris/tests/test_quickplot.py
@@ -1,4 +1,4 @@
-# (C) British Crown Copyright 2010 - 2015, Met Office
+# (C) British Crown Copyright 2010 - 2016, Met Office
 #
 # This file is part of Iris.
 #
@@ -60,6 +60,7 @@ def _load_theta():
 @tests.skip_plot
 class TestQuickplotCoordinatesGiven(test_plot.TestPlotCoordinatesGiven):
     def setUp(self):
+        tests.GraphicsTest.setUp(self)
         filename = tests.get_data_path(('PP', 'COLPEX', 'theta_and_orog_subset.pp'))
         self.cube = test_plot.load_cube_once(filename, 'air_potential_temperature')
 
@@ -110,6 +111,7 @@ def setUp(self):
 @tests.skip_plot
 class TestLabels(tests.GraphicsTest):
     def setUp(self):
+        super(TestLabels, self).setUp()
         self.theta = _load_theta()
 
     def _slice(self, coords):
@@ -184,8 +186,8 @@ def test_alignment(self):
 @tests.skip_data
 @tests.skip_plot
 class TestTimeReferenceUnitsLabels(tests.GraphicsTest):
-
     def setUp(self):
+        super(TestTimeReferenceUnitsLabels, self).setUp()
         path = tests.get_data_path(('PP', 'aPProt1', 'rotatedMHtimecube.pp'))
         self.cube = iris.load_cube(path)[:, 0, 0]