iTUNES podcast design?

I know this is pretty easy, but I can’t seem to find it…

how can you [edit]: upload a design for a podcast page

these links help but they only show how to create a .jpg thumb

http://www.makezine.com/blog/archive/2005/07/how_to_make_enh.html

http://www.mvldesign.com/itunespodcast.html