In practical terms, if you don't formally register your images with the copyright office, if someone infrignes and you take them to court, you can pretty much only hope to keep them from using it any more. While in theory you can collect up to a maximum of $150,000 per copyright violation, this almost never happens, and collecting any noteworth amount in damages is really only possible if you have a formally registered copyright.

Read up on the EP, APA, and ASMP web sites and buy a copy of the ASMP Business Practices book. When you find someone stealing your work, you won't regret having done so.