using md5sum to avoid CP

Say every time some dude post CP, in addition to permaban the punk, you make a md5sum of the pic (or of several parts of the pic - wiser I think) and store the md5 hash in a database table checked for each pic posted on a board. Obviously even a tiny modification of the pic would make the script useless. Then it could be better to analyze different parts of it. Since the same pics are posted over and over, It could be useful as an autoban feature and pretty easy to write. Obviously it will fail if the pic is cropped or resized.