Digital image manipulation software is readily accessible from the Internet today, thus, digital data is very easy to be tampered without having any clue. Under this circumstance, integrity verification has become an important issue in the digital world. The aim of this paper is to present an in-depth analysis on the methods of detecting image tampering. We introduce the notion of image content authentication and the features required to design an effective authentication scheme. We review some algorithms and frequently used security mechanisms. We also analyze and discuss the performance trade-offs and related security issues among existing technologies. Compared with those techniques and protocols based on digital signatures or watermarking, majority of the proposed image hash methods emphasize more on content authentication rather than strict integrity.