Digital watermarking is an effective technique to protect the intellectual property rights of digital images. In general, a gray-level image can provide more perceptual information; moreover, the size of each pixel in the gray-level image is bigger. Commonly, gray-level digital watermarks are more robust. In this chapter, the proposed watermarking scheme adopts a gray-level image as the watermark. In addition, discrete cosine transformation (DCT) technique and quantization method are applied to strengthen the robustness of the watermarking system. Both original image and digital watermark, processed by DCT transformation, can build a quantization table to reduce the information size of the digital watermark. After quantized watermark is embedded into the middle frequency bands of the transformed original image, the quality of the watermarked image is always visually acceptable because of the effectiveness of the quantization technique. The experimental results show that the embedded watermark can resist image cropping, JPEG lossy compression, and destructive processes such as image blurring and sharpening.