Oh yeah been down this road more than once, the biggest was last year, which was so bad I quit weddings and just do portraits now. Somebody took the watermarks off and posted on myspace. Needless to say it went to court and I got financial rewards, but after that I was done with weddings and posting images online. Now, during a portrait session, they view the proofs right there, I take the order, done. Any future orders come through me too. Its unfortunate, but it saves me in the long run. I second that website
TinEye Reverse Image Search amazing site.