Yeah, let the admin choose to either put up a default watermark on every image that's uploaded (except registered users, or users from a certain usergroup...=> for later when a registered user option is available) or to allow the uploader himself to put a watermark text or image on it
Location is of key importance here and if text watermark, text options (color, font, size, ...)
Also let a user upload an image specifically for watermarks (=> for later when a registered user option is available)