This discussion came up on another forum, and a link to a free service at http://tineye.com/
was posted there. I signed up to try the beta version, which put me in the que to be signed up after receiving a request confirmation via email. (It took less than 24 hours to receive the actual invitation.)
The concept is that Tin Eye will search the web for pixel matches, even after the image has been altered in Photoshop.
I think this is a "great" idea, and hope that someday this will put an end to unscrupulous individuals profiting from the works of others. 
They have a neat plug in for Firefox that allows you to right click on an image where the menu will now include a search option for that image.
I'm now one of the Beta testers, but their number of listed sites has to grow to improve their data base. I've submitted ebay for starters. 
Anybody else heard of them?
-Bruce






