Package libpuzzle

Library to quickly find visually similar images (gif, png, jpg)

http://libpuzzle.pureftpd.org/project/libpuzzle

The Puzzle library is designed to quickly find visually similar images
(gif, png, jpg), even if they have been resized, recompressed,
recolored or slightly modified. The library is free, lightweight yet
very fast, configurable, easy to use and it has been designed with
security in mind.
System Administration (Section 8)
puzzle-diff
puzzle-diff compares two pictures and outputs the normalized distance. Try puzzle-diff -h for more info.