My suggestion is to integrate watermarking into one of the features in the Media Library.
The user could select between 2 additional options on upload:
Permanent watermarking - New versions of uploaded image are created, the unwatermarked original file is not kept on server ( this would save tons of server space if you are uploading 20+ files a day as some of my photo blogs are setup to do ).
Non-destructive Watermarking - Save the unwatermarked original and only display the watermarked to site viewers / in the rss feeds. For the win - the original file could be stored in a non-spiderable/http downloadable folder. This would prevent unwanted downloading and/or linking to the originals.
I am monitoring the forum for the term "watermark" and there is quite a few threads popping up with people stuck on watermarking related issues. I think a watermarking feature has the potential to benefit millions of users. I will definitely do my best to help !!