Not sure what you mean by "default white background"; most of the banners have black backgrounds.
In Photoshop, I think the easiest thing to do is create a new file with a transparent background, paste in the banner, and delete the black background around it. (Simplest way is with the magic wand selection tool.)
Yeah. I didn't understand it and now I do. Sounds easy enough. If we can get a couple people to do it, I think we can finish it in a week.
