Of course even something like that's not going to survive a crop or someone going in and removing the image tags (as you noted). To really do something like this you'd want to look into something like Photoshop's watermarking plugin that's much harder to remove. It depends on how determined you are to mark the image (and how determined the swiper is to swipe ("Swiper no swiping!"</Dora the Explorer>; yes, I have young children why do you ask? :)).