The more well known an artist gets, the more their work is circulated around the net. Eventually things get ripped, stolen, misrepresented, copied, ect, especially if that work has a mass appeal to it.
While its annoying, there doesn't seem to be a legal solution that makes sense to the artist unless they have a reason to justify a tedious and involved court battle over rights. USUALLY people aren't making enough off a stolen work for this to be a realistic solution.
also, someone brought this site up in a recent poll: [link] its down right now, but it identifies images online based on similar visual quality. Seems like a good tool to catch stolen work.
I think the best thing to do as an artist is to clearly represent yourself and your work online. That way art is easily traced back to your website, instead of floating around without an author, or some place to source it from.
I've had an image traced, but thats about it as far as i know. It was done by one of my long-time watchers who fav'd almost everything I uploaded :/ I didn't really do anything about it... I let them know that I saw it but I didn't tell them to take it down. They updated the description to credit me though, so everythings cool.
When i used to go on gaia people would steal my art. Someone went so far as to make their name like mine then post my pictures pretending to be me. Then someone took my dress up game and was selling poeple screen caps of the outfits for gaia gold. Rediculous.
I've had my art copied (not so bad), completely traced over (annoying), parts repainted over the top of with other parts left as-is (even more annoying), reposted completely, as-is, with my signature removed and the person claiming it as theirs (VERY ANNOYING) and put onto clothing at IMVU and sold for profit.