You altering images does not give you any form of copyright.
How much do I have to change in order to claim copyright in someone else's work?
Only the owner of copyright in a work has the right to prepare, or to authorize someone else to create, a new version of that work. Accordingly, you cannot claim copyright to another's work, no matter how much you change it, unless you have the owner's consent. See Circular 14, Copyright Registration for Derivative Works.
http://www.copyright.gov/help/faq/fa...se.html#change
Also, putting hours of work into something and getting paid for it does not grant you some sort of copyright protection. Now, if they ripped the style in which you presented it, and had a clear copyright ownership, that would be something to fight about. Seeing as it's a, collaboration between community members for a game, I'm guessing it would fall somewhere around creative commons licensing, unless you have somewhere on your website a stated license in which you released the content.
I get that you're all, hurt and stuff, you have every right to be, but the truth is it doesn't look like you have much of a legal leg to stand on. Now, the thought of legal action might cause them to be proactive just, because, but that doesn't mean you have a right to bring it against them, or that it would hold up in court.