1. A method of affixing a covert digital identifying indicia to a digital image, comprising the steps of: providing, to a processor, an input image defined by an array of pixel units each pixel unit having a control quality selected from the group consisting of luminance and saturation; providing to said processor an identifying indicia template defining an identifying indicia image and formed of a plurality of bits which have a spatial correspondence with pixels in said array of pixel units; altering, in said array of pixel units defining said input image, the control quality of pixel units which have said spatial correspondence, to a pre-selected difference level, said pre-selected difference level being in a range selected to render the alter ed pixel units minimally detectable to the human eye when they are displayed; generating, from said processor, an array of pixels defining an output image, said array of pixels in said output image being comprised of said input image with said altered pixel units, so as to embed in said output image covert detectable identifying indicia which represent said identifying indicia image.