1. A method for detecting a copy of a digital image, comprising: dividing a suspected copy of a digital image into sub-images; determining average intensities of pixels associated with each of the sub-images; transforming the average intensities into a series of coefficients; defining a rank matrix from the series of coefficients; and comparing the rank matrix from the series of coefficients to a rank matrix of a query image to determine if the suspected copy is an actual copy of the digital image.