I always trace the domain to their host and email their host with the subject "violation of copyright on one of your hosted web sites" and then explain that I have found my material (pics etc) on the site and it is in clear violation of my copyright. I have had to do this a dozen times and in half the cases it resulted in the web site being pulled down because they either couldn't get hold of them or they wouldn't take it off.
Web hosts take copyright theft very seriously because they can become liable if nothing is down about it. Most try to deal with it within 48 hours, they treat it that serious.
I've had one client (tourism) that I did a web site for and he refused to pay me. He set up a new web site using MY site. It was exactly my site right down to my images and meta tags which still had my company name on them. The client managed to blag his host to keep it on so I was told there was nothing I could do about it... WRONG! later that day I registered a www.******hell.com and uploaded the site I created on to it a submitted it everywhere explaining that the company didn't pay us and he stole my work. If you found his site you also found my site.
Guess who paid up :)