"There are two artists on Artwanted whos images are being used without permission on products and websites and these companies won't stop. One is getting "undeliverable message" and her image is being used as a puzzle...."
Where they are unable to contact the site owner, they will need to go to the hosting service and alert them. They'll need to look up the main domain on a whois (http://www.samspade.org) and go through the company whose servers it's hosted on. In most cases, where there is no valid site owner contact info, they will kill the account until the owner contacts them.
Hope it helps...I think this is close to the last tip, but will help you get them