1. A method of determining whether an acquired image matches a reference image, comprising: acquiring a digital image for comparison to a reference image; identifying a group of pixels in the acquired image; selecting the pixel group in the reference image that is the best match for the pixel group in the acquired image; repeating the preceding steps for at least one more group of pixels; comparing the relative locations of the selected pixel groups in the reference images and the pixel groups in the acquired image; and determining the probability that the relative locations occurred randomly.