Came across this site today and think they might be onto something: http://tineye.com/
Here's a real world example of how they used their search engine to detect fraudulent photo usage: http://blog.ideeinc.com/2009/06/02/fakorama/
It's still in it's infancy and I did a search on the Spanky photos that were borrowed from the thread in the General Discussion forum and it did not return any hits. :-( Perhaps in due time? Even so I installed the FireFox add-on, so my curiosity is just a right+click away.