We see alot of people bring in low res files to our print shop to try and have them printed. Usually we don't do the work, because they're just too small or low resolution, not to mention, that if they have very low resolution files, it's probably because they stole them from the internet somewhere, and I don't really approve of that. Either way, the point is, most print shops won't print very low resolution files, so even if someone did steal a low resolution file, they couldn't do much with it, or ruin your life too badly.
Avoid watermarks, because they really can detract form your image as a whole - just stick with low res files - about 6"x8" at 72dpi should be low enough that prints can't be made from them, but still a reasonable size when viewing on the web.
Good luck!