With the rapid growth of the Internet and the popularity of digital imaging devices, digital imagery has become our major information source. Meanwhile, the development of digital manipulation techniques employed by most image editing software brings new challenges to the credibility of photographic images as the definite records of events. We propose a set of effective tools using local signal statistics to address two critical digital image forensic problems: (1) Detecting region duplication; and (2) Exposing splicing forgery. Based on a local noise level estimation algorithm developed for image splicing detection, we also explore several other important applications: (i) Exposing special visual effects; (ii) Medical image denoising; and (iii) Detecting audio splicing. For each tool, we report experiments that demonstrate the effectiveness of the proposed methods both quantitatively and qualitatively. [The dissertation citations contained here are published with the permission of ProQuest LLC. Further reproduction is prohibited without permission. Copies of dissertations may be obtained by Telephone (800) 1-800-521-0600. Web page: http://www.proquest.com/en-US/products/dissertations/individuals.shtml.]