go to
http://www.custom-cgi.com/free and download the Ace Bandwidth Protector. It's a script to run your images through and has a set .gif file that is sent whenever someone tries to pull your images. Pretty easy to set up, especially if you have a multi-replace program. You can make the .gif file anything you want.. or send html instead.. you could really mess with the thieves with that..
Just imagine plastering your own big ass add to their pages....