Finding similar images is not at all a trivial task. Entire PhD dissertations have been committed to it. The solutions are still very unreliable as of yet. If you want to find more, you can read the research out of the ongoing Image CLEF track. I worked with them briefly a couple of years ago in context of medical images.
http://ir.shef.ac.uk/imageclef/ -- http://mail.python.org/mailman/listinfo/python-list
