All cameras introduce varying degrees of imperfections. While these imperfections are undesirable for the consumer, they can be highly informative in an image forensic setting. Sensor imperfections, thermal effects, compression, etc. each introduce noise in the image. This noise manifests itself as slight variations in the recorded intensities of light. If a portion of an image is altered then it is likely that the underlying noise pattern will be disrupted. Differences in the noise pattern can therefore be used to detect and localize image tampering.
via Image Authentication and Forensics | Fourandsix Technologies.