Any image you create....
...have it registered with with copyright office. Its only cost 25 bucks (a copy of your images on cd) and a few moments of your time. Once registered. You can sue for "Federal Copyright Infringement". Which means they pay 3 times the amount of damages plus your attourney's fees included. If you don't register it with the copyright office then its your word against his...and a judge will just laugh at you.
If you don't have it federally copyrighted...then it becomes a civil court case. Which means you can only sue for damages. Plus you have to pay your own attourneys fees. Most likely it will cost more to sue and lawyers fees then you would get back in damages.
Case in point, ALWAYS sumbit your work to the copyright office first. Then let those basterds steal your work. Then sue him/her in FEDERAL Court...and bam...you looking at 2,000 to 3,000 dollars or more and the offender pays all your fees.
Here is the link to the copyright office...(fill it out now).
http://www.copyright.gov/
Sue him/her in 6 months. Have screen shots of all offenses. Get all there online addresses. Then send a certified letter through an attourney. They will have no way out.