Eh being an artist, I’ve had times were artwork (sometimes entire series of work) I’ve done has been claimed by other people as work they’ve done. Sure it’s sucks, but it the nature of the internet, you’re not going to get a jerk free enviroment, so I have little sympathy for those who whine about it. I knew when I published it to the net there was the possibility some idiot would say it was him, and that it would be hard to refute him across the internet. (Other than when the idiot was dumb enough to be hotlinking the image from my webspace, so I’d change it to say, a picture of someone in surgery for fun.)
Hell I know artist who go onto foriegn boards just to find there are in some other language where they can’t possibly try to adequately tell the people the guy’s a liar, and while it sucks, it’s not something that anyone should be all that surprised about.
Usually there is a few ways to go about it, I’ve had friends who print their URL across there image, so while it hurts the overall look of the art, it makes it painfully tedious to steal. I’ve seen people close down shop all together, thus assuring that no one steals there work online. (Really expecting the entire populance of the internet to be civil when internet society is worse than the already less than civil real world, is just an exercise in futility. Ideally, that’s how the world would be, but you have to be realistic sadly.) I’ve also seen people who said the hell with it, and kept posting art anyway.
The way I do it is simple, if it’s important to me, I’m not putting it up to be dissected on the net, I’ll run a print run and sell it, that way I’m sure the product that reaches people is the one I want, otherwise people can, and sadly, will alter it if they so please.
For mediums that cannot be protected that way, like creations by mugen enthusiasts, then you’re going to have to be able to put up with it, because there are people who will claim your work, and they don’t really care how upset you get about it.