I wasn't sure whether to call this an addon (since it doesn't touch the index/admin code) or a mod (since it can't live in /addons), but I've just added a script & .htaccess file that generate a watermark on the fly for every photo in the /images directory.
Just create a watermark PNG, upload it & wrapper.php to /admin, then edit & upload .htaccess to /images.
I didn't actually write it, and don't like what on-the-fly image generation does to server loads, but it's a nice stopgap until I have time to write a real mod.
Mostly, I'm posting to find out why I couldn't add a screenshot to my entry in the developer's system? It says it's there, but doesn't show up...