I think in the internet, art theft simply is a topic.
However, I think uploading low-resolution files (or rather: with small size, not with small quality) would be an idea. I personally do that for a while now, cropping my picture to about the size I want to upload it, and maybe only make it one step smaller by DeviantArt. Using the DA watermark would be an idea, though I'm not sure how well that works - I guess one can photoshop it out quite easily.
Another option would be to upload things in "small size", and to cover it with watermark-lines (I saw this here, for example: A Fox Named Fable
). The artist in the sample made this very well I think, as the writing doesn't "disturb" the picture itself.
Also, there are pages where you can register your artwork, like saying "this is mine". I can't find the names of them right now; however, some of them also charged money for their service, or only had so-and-so many artworks registered for free.