This article reveals technology that could prove that an image was taken by a cerain camera based on the "fingerprint" of the camera:

In technical terms, Fridrich and her co-inventors Jan Lukas and Miroslav Goljan found that every digital picture is overlaid by a weak noise-like pattern of pixel-to-pixel non-uniformity. As with fingerprints, that digital noise pattern will be consistent among all images taken from the same camera.

Though this technology is intended to be used against child pornography (as it exaplained in the article), I think that the same technology could be used to prove the copyrights of photographers on their photographs. The only thing is that the cameras should be legaly registered (the fingerprints scanned). So later on when the camera changed the owner, you still can prove that an image was taken by your camera. Well, you got the idea.