If you're adding a watermark to prove authenticity of an image, could i suggest doing a super for steganography?

If you use an MD5 of something or other and insert it into the image, all you have to do to prove the image is authentic (and hasnt been modified) is pull out the embedded message and check it.

OK, so its a little more complicated than that, but not much more. The mechanism for computing a message is pretty easy using MD5, and bit fiddling a jpg is nice and easy with pack. I guess the considerations are what you will use as the key to create an MD5 (or SHA-1), and where to store it, as well as the automation to do so.

But i may be off-topic here: If you're using the watermark to degrade an image so people dont use them, this isnt probably the best solution.. :-)