If you are going to use a watermark, care should be taken to ensure that the watermark is effective, not only for proving that an image was modified, but also for educating the would-be pirates about copyright and ownership. Ideally, the watermark should be a copyright symbol along with the name of the owner, and the URL of the owner's web site, if applicable. This not only conveys the message of copyright, but it gives others an opportunity to contact the original owner of the image.