1. A method for processing mammographic images comprising the steps of: forming in a first mammography system a digital or digitized mammogram of a breast along with images of first and second reference materials having thicknesses that range from 0 to the thickness of the breast, one reference material having an attenuation constant that is approximately the same as that of fat and the other having an attenuation constant that is approximately the same as that of glandular tissue; using exposure information in the images of the first and second reference materials to process the digital or digitized mammogram system to transform the digital or digitized mammogram into a first processed image whereby substantially all effects related to the physical characteristics of the first mammography system and its operating parameters and the effect of fat content in the breast being imaged are removed; transforming the first processed image into a standard-form mammogram having pixel values that would have been obtained by a standard-form mammography system having a first standard x-ray voltage parameter and a first standard exposure parameter; and storing said standard-form mammogram whereby visual comparison of mammograms taken by different mammography systems is facilitated by comparing standard-form mammograms derived from mammograms taken by the different mammography systems.