Just today I got the idea to try and search for my own images via Google image search. And only now have I realized how many times were my pieces stolen and used in pages/games/posters etc. without any permission and they did'n even bother to give any credit. It's really disgusting but it looks like it's unavoidable, especially if you upload your work without those watermarks so people can fully enjoy your work.
I did send tons of e-mails today and I recommend you try out the image search by Google on your pieces as well, you might get surprised.
P.S. They actually have the nerve to watermark our stuff with their shit.