The unique position of photos as a definitive record is being undermined due to the ease with which digitally produced imagery can be manipulated.
Even though competent forgeries might not leave any visual clues of tampering, they may alter the under-lying statistics of a image. Re-sampling (for example scaling / rotating) produces particular statistical connections, and these connections can be auto-detected in a portion of the image.
This functions even when a digital watermark or signature is not present. This technique is the first of many tools that will be needed to expose digital forgery.