In this paper, we shall propose an approach to protect the copyright of image data. There have been many watermarking techniques proposed to guard the copyright of image data, most of which need to embed the watermark in the cover image. Unfortunately, the embedding always results in the reduction of image quality. To achieve the goal of copyright protection without reducing the image quality, we incorporate the concept of digital signature into our approach. At the beginning, we produce a signature in correspondence with the cover image and the watermark. We send this signature to a certification authority or a trusted third party. If someone pirates our image someday, we can still retrieve the watermark image with the signature that was stored at the certification authority and prove that the image is ours. Notice that the watermark image that we use in this work is a gray-level image.