Well, I have thought about this, and for me it seems complicated at the moment.
It means each image uploaded should pass through the "watermarking", but there are quite a lot of image types allowed. GIFs, BMPs, JPGs... and this "watermarking" thing would work only for JPEG. In other case, all uploaded images should be converted to JPEG, losing their original quality and type.
You could take Photo album add-on, where watermarks are implemented, and you will see it uses only JPG files. I'm not sure if the same thing is suitable for Image attachments add-on. It's purpose is different.
So we will skip this implementation for now. Not a $15 question. 10x bigger I must say ;-)