We propose an approach to protect the copyright of image data. Many watermarking techniques have been proposed to guard the copyright of image data, most of which need to embed the watermark in the cover image. Unfortunately, embedding always results in a reduction of image quality. To achieve the goal of copyright protection without reducing image quality, we incorporate the concept of digital signature into our approach. Firstly, 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 anyone pirates our image, we can still retrieve the watermark image with the signature that was stored at the certification authority and prove that the image is ours. The watermark image that we use is a gray-level image.