The best advice is to read the FAQ thread her eon the Tech Board. There is an excellent clip art primer that explains it very well.
The very condensed version of the answer is that you have to put the picture on a website and then use HTML code in your signature to reference the picture. The difficult part is to find a Website that will allow you to 'hot link' a photo from their site into another website (which is the Dis Boards here).
hope that helps