Put your name on it and a watermark, like bonni already explained. If you have your work stolen and tubed with a different name and copyright plasterd all over it and shared on various Yahoo Groups (like some of mine) you are starting to make the watermark as large and visible and heavy to remove as possible. A small copyright line somewhere at the border of the image can be cropped away. Make it visible and hard to remove.

Every organisation rests upon a mountain of secrets ~ Julian Assange