first working version.

.gitignore

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+pkg/*
+*.gem
+.bundle

Gemfile

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+source "http://rubygems.org"
+
+# Specify your gem's dependencies in jekyll-minimagick.gemspec
+gemspec

Gemfile.lock

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+PATH
+  remote: .
+  specs:
+    jekyll-minimagick (0.0.1)
+      jekyll (>= 0.10.0)
+      mini_magick (>= 3.3)
+
+GEM
+  remote: http://rubygems.org/
+  specs:
+    classifier (1.3.3)
+      fast-stemmer (>= 1.0.0)
+    directory_watcher (1.4.0)
+    fast-stemmer (1.0.0)
+    jekyll (0.10.0)
+      classifier (>= 1.3.1)
+      directory_watcher (>= 1.1.1)
+      liquid (>= 1.9.0)
+      maruku (>= 0.5.9)
+    liquid (2.2.2)
+    maruku (0.6.0)
+      syntax (>= 1.0.0)
+    mini_magick (3.3)
+      subexec (~> 0.1.0)
+    subexec (0.1.0)
+    syntax (1.0.0)
+
+PLATFORMS
+  ruby
+
+DEPENDENCIES
+  jekyll (>= 0.10.0)
+  jekyll-minimagick!
+  mini_magick (>= 3.3)

Rakefile

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+require 'bundler'
+Bundler::GemHelper.install_tasks

jekyll-minimagick.gemspec

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+# -*- encoding: utf-8 -*-
+$:.push File.expand_path("../lib", __FILE__)
+require "jekyll-minimagick/version"
+
+Gem::Specification.new do |s|
+  s.name        = "jekyll-minimagick"
+  s.version     = Jekyll::Minimagick::VERSION
+  s.platform    = Gem::Platform::RUBY
+  s.authors     = ["Roger López"]
+  s.email       = ["[email protected]"]
+  s.homepage    = "https://github.com/zroger/jekyll-minimagick"
+  s.summary     = %q{MiniMagick integration for Jekyll}
+  s.description = %q{Use MiniMagick to crop and resize images in your Jekyll project.}
+
+  s.rubyforge_project = "jekyll-minimagick"
+
+  s.files         = `git ls-files`.split("\n")
+  s.test_files    = `git ls-files -- {test,spec,features}/*`.split("\n")
+  s.executables   = `git ls-files -- bin/*`.split("\n").map{ |f| File.basename(f) }
+  s.require_paths = ["lib"]
+
+  s.add_runtime_dependency('jekyll', [">= 0.10.0"])
+  s.add_runtime_dependency('mini_magick', [">= 3.3"])
+end

lib/jekyll-minimagick.rb

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+require 'mini_magick'
+
+module Jekyll
+  module JekyllMinimagick
+
+    class GeneratedImageFile < Jekyll::StaticFile
+      # Initialize a new GeneratedImage.
+      #   +site+ is the Site
+      #   +base+ is the String path to the <source>
+      #   +dir+ is the String path between <source> and the file
+      #   +name+ is the String filename of the file
+      #   +preset+ is the Preset hash from the config.
+      #
+      # Returns <GeneratedImageFile>
+      def initialize(site, base, dir, name, preset)
+        @site = site
+        @base = base
+        @dir  = dir
+        @name = name
+        @preset = preset
+      end
+
+      # Obtains source file path by substituting the preset's source directory
+      # for the destination directory.
+      #
+      # Returns source file path.
+      def path
+        File.join(@base, @dir.sub(@preset['destination'], @preset['source']), @name)
+      end
+
+      # Use MiniMagick to create a derivative image at the destination
+      # specified (if the original is modified).
+      #   +dest+ is the String path to the destination dir
+      #
+      # Returns false if the file was not modified since last time (no-op).
+      def write(dest)
+        dest_path = destination(dest)
+
+        return false if File.exist? dest_path and !modified?
+        @@mtimes[path] = mtime
+
+        FileUtils.mkdir_p(File.dirname(dest_path))
+        image = ::MiniMagick::Image.open(path)
+        image.resize @preset['resize']
+        image.write dest_path
+
+        true
+      end
+
+    end
+
+    class MiniMagickGenerator < Generator
+      safe true
+
+      # Find all image files in the source directories of the presets specified
+      # in the site config.  Add a GeneratedImageFile to the static_files stack
+      # for later processing.
+      def generate(site)
+        return unless site.config['mini_magick']
+
+        site.config['mini_magick'].each_pair do |name, preset|
+          Dir.glob(File.join(preset['source'], "*.{png,jpg,jpeg,gif}")) do |source|
+            site.static_files << GeneratedImageFile.new(site, site.source, preset['destination'], File.basename(source), preset)
+          end
+        end
+      end
+    end
+
+  end
+end

lib/jekyll-minimagick/version.rb

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+module Jekyll
+  module Minimagick
+    VERSION = "0.0.1"
+  end
+end