1. A computer-implemented method for detecting whether a suspect image has been digitally altered, the method comprising: identifying a set of quantized DCT blocks that represents the suspect image, each quantized DCT block including a plurality of AC modes and each AC mode corresponding to one of a plurality of quantized DCT coefficients, the plurality of quantized DCT coefficients having a one-to-one correspondence with the plurality of AC modes, each AC mode being located at a consistent position within each of the quantized DCT blocks in the set; computing a first digit for each of the quantized DCT coefficients corresponding to one of a pre-determined number of AC modes in each quantized DCT block; creating a feature vector using the first digits computed for each quantized DCT block, the feature vector representing a first digit distribution over the pre-determined number of AC modes for the suspect image; inputting the feature vector to a set of classifiers; and determining a result for the suspect image based on an output from the set of classifiers.