1. A method for applying a digital watermark to output images from a computer program, comprising: generating, for each of the output images, the digital watermark, the digital watermark having at least one attribute, wherein each attribute is a function of a variable associated with the attribute, the variable being capable of modification from image to image to produce a different digital watermark on each of the output images; and applying the digital watermark to each output image to produce corresponding watermarked images.