1. A transmitter for compressing and transmitting a digital image over a limited bandwidth communication channel comprising: (a) means for performing a two-dimensional spatial frequency transformation on the digital image to produce transform coefficients; and (b) means for quantizing the transform coefficients in accordance with a two-dimensional model of the sensitivity of the human visual system to spatial frequencies, said model of the human visual system including the feature that the human visual system is less sensitive to diagonally oriented spatial frequencies than to horizontally or vertically oriented spatial frequencies.