1. A method of encoding a digital image to permit its later identification, the image comprising plural pixels, each having a value, the method comprising: receiving an N-bit data string to be embedded in the image, N being at least two, the string comprising bits having first and second values, one of said values being “1” and the other of said values being “0”; associating a first bit of said string with a first pixel, a second bit of said string with a second pixel, and so on, so that each bit of said string has a pixel associated therewith; for bits having the first value, changing the values of pixels associated therewith, such changes being essentially imperceptible to a human viewer of the digital image; leaving the values of most of the pixels in the image unchanged; storing the N-bit data string for later reference.