I suspect someone will have a better answer than the one I'm about to give, but I think the trick is to gave the graphic in a format that allows for you to have a separate alpha channel. When you bring your graphic up in Photoshop or Paint Shop Pro or whatever you are using to do your postwork, you can delete the background using the alpha channel. I don't believe (though I could be wrong) that there is a way to render on a transparent background.