My suggestion is to add an option, on upload, to add a custom water-mark to an image.

Maybe a text-box that appears when you click an check-box labled "Add watermark to image"

The water-mark could be placed in the bottom-right of the image with a standard font.

(I know you can do this in image-editing programs)

Also, it would be nice if this was to happen, to allow watermarks to be added to images already uploaded.