I did one recently and invented my own method. I was using Paint Shop Pro. I set up a very large airbrush tool set to spray the background color the image would go on (black, in that case), and I sprayed as close as I dared without touching the outline of the image. Then I magnified the image and went around the ouline with progressively smaller airbrushes. In the end, I used a solid paintbrush to push the background color right up to the edge of the image. Maybe it's not the greatest knockout job ever done, but it looks okay, I think (
http://www.girlsdotcom.com/tour0.htm).