https://www.pureftpd.org/project/libpuzzle
This link is now broken.
about:
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. There is a PHP extension.
