A visible digital watermark is applied to output images from a computer program. Various attributes of the watermark are modified from image to image, making the watermark difficult to remove. For example, a watermark indicating "not for commercial use" can be applied to all output images from the program. Such watermarking permits full functioning demonstration versions of the computer program to be freely distributed to users while commercial use is inhibited. This is accomplished by a minimal level of intrusiveness to file sharing capabilities between a commercial version of the software and a non-commercial version of the software.