1. A method of determining whether a digital image is either a computer graphics image or a digital photographic image, the method comprising: extracting a first set of features from an input digital image, said extracting comprising: applying a discrete transformation to decompose the input digital image into a set of sub-bands; forming characteristic functions associated with the sub-bands and with the input digital image; and obtaining sets of moments based at least on the characteristic functions associated with the sub-bands and with the input digital image, wherein said sets of moments are used to form said first set of features; extracting a second set of features from a prediction-error image derived from said input digital image; and applying a classification algorithm to said first set of features and said second set of features to determine if the combined sets of features indicate that the input digital image is a computer graphics image or is a digital photographic image.