A free program called GIMP will do what you want, it is the open source alternative
to photoshop... you'll probably have to figure out how to use "layers" and some other
non-intuitive stuff about transparencies and alpha channels to do what f6gal did, so
there's some amount of learning curve, but not too hard after you know what you're
trying to do...
http://en.wikipedia.org/wiki/Alpha_compositing-Mike "I don't understand anything after the 1st couple of paragraphs on the wiki page"