Clearing the image cache makes static resized images show up as missing

[bennothommo]

@bradietilley I encountered this on Summerstar.

If an image is using the permalink storage method for resized images, and a dev elects to clear the image cache, it appears to be removing these static images correctly - however, upon refresh, the static images are showing the placeholder image.

I believe this is because the permalink records are still stored in the DB and pointing to a file that no longer exists - the permalink functionality basically isn't recognising that file is not available anymore and regenerating it, it simply goes straight to showing the placeholder image.