Got you.
There’s no way you could practically pre-process for that.
You either have to watermark on the server, when serving the images (blimey, hope you’ve got deep pockets for an almighty computer, even if that were possible).
OR
One could try and watermark on the javascript layer … I’m not sure how easily that would integrate … can’t image the performance of that would be good … and it would be insecure and open to hacks (because you’d have had to serve up the originals)
Perhaps someone here has experience of doing exactly this?