1. An image digest processing device, comprising: a transformation unit that generates a frequency-based representation of a received image; an approximation unit that generates a nonnegative matrix approximation of the frequency-based representation of the received image; a digest unit that generates an image digest based on the nonnegative matrix approximation of the frequency-based representation of the received image; an image repository that includes a plurality of image digests, one or more of the plurality of image digests stored in association with a separate one of a plurality of images stored in the image repository from which each of the respective image digests was generated; a search unit that performs a search of the image repository based on one or more image digests generated for the received image; wherein the search unit determines a distance between the one or more generated image digests and one or more image digests associated with an image in the image repository and identifies the image in the image repository in a search report if a distance is less than a predetermined threshold value; a threshold determining unit that determines a predetermined threshold value used by the search unit; wherein the threshold determining unit determines the predetermined threshold value based on an analysis of an image repository to be searched; wherein analysis of an image repository to be searched includes determining a probability of false returns and a probability of misses associated with the predetermined threshold value; and notifying a user of the probability of misses and the probability of false returns, and prompting the user to manually set an upper bound for the probability of misses and upper bound for the probability of false alarms.