The great success of internet and the ease of copying and distributing the electronic data has presented a new challenge of how to protect the digital data. Digital watermarks have been proposed as a means for copyright protection of multimedia. Unlike the traditional visible watermark, the challenge here is to introduce a digital watermark that does not alter the quality of electronic content, while being extremely robust to attack. From the angle of signal processing, the two basic requirements for an effective watermarking scheme, robustness and transparency conflict with each other. So the digital watermark is information that is imperceptibly and robustly embedded in the host data such that it can not be removed without important degradation of images quality.\; This paper consists of two parts. In the first part, an authentication technique by embedding digital “watermarks” into colored images is proposed. In our approach, we embed the “watermarks” with visually recognizable patterns into the images by selectively modifying the middle frequency coefficients of the image. In the second part, the technique of hiding a colored image into another colored one is proposed. The experimental results show that the proposed techniques successfully survive image processing operations, image cropping and the JPEG lossy compression.
