||As the traditional methods of data hiding and hardware control cannot effectively help in protecting the copyright of media files which are widely and easily distributed through the internet, embedding imperceptible information into the files has drawn the attention of many researchers and media file owners. Precisely, the focus is on embedding the information into the media files. The hidden information has to be robust against intentional attacks, transmission and compression distortions and remains visually or auditory imperceptible. The embedded information is called watermark. In this thesis, we describe the common requirement and definition of a watermarking scheme, summarize the previous works and propose an image certificate system which can verify the ownership of still images. The new system can protect the non-invertibility of the watermarking schemes used.