1. A method of watermarking an image to facilitate detection of copying of the image, the method comprising: (a) providing image data that represents an image, the image data comprising pixel data that represents respective gray-scale values of pixels of the image; (b) forming a plurality of data blocks from the image data, each data block consisting of pixel data which corresponds to a respective region of the image; (c) determining for each of the data blocks an average value of the pixel data in the data block; (d) determining for each of the data blocks a target for the average value of the pixel data in the data block; and (e) adjusting respective values of at least some of the pixel data in each of at least some of the data blocks to shift the average value of the pixel data in the respective data block toward the target for the respective data block.