I wholeheartedly agree with everyone else. Stolen property is stolen, no matter how big or how small. It may cost you more than seems worth it but you will be satisfied in the end. Don't let people bully you around! I love that piece of artwork and It makes me mad to think that someone else claims it for their own >(
I am a law school student. If you file suit against them for un-warranted usage of intellectual property, there is no judge that will rule in favor of the other party. At the least they will have to 1. Cease all production. 2. Continue production with a percentage of sales going to you, 15% may sound like shit, but its about all you can get for a design. At the most, you can get either of these and restitution for illegal usage.