in reply to BMP: Drawing or photograph?
Just pull a histogram on a random sample of the pixels, then measure the standard deviation. If it's low, i.e. if the histogram is well-distributed between highs and lows, and especially among colors, it's a photograph. If it's high, and tends toward white or black pixels, it's a drawing. Asssemble a test set of a few hundred of both types for algorithm-training purposes.