Digital watermarking is one of the most widely used techniques for protection of ownership rights of digital audio, images and video. Its commercial applications range from copyright protection to digital rights management. The success of a digital watermarking technology depends heavily on its robustness to withstand attacks that are aimed at removing or destroying the watermark from its host data. This paper provides analysis of a number of digital image watermark attacks and attempts to classify them into categories. A set of experimental results are also provided to show the effect of these attacks on watermarks produced using different watermarking techniques.