You can edit the text, but not add pictures. They changed this about a year ago. I exchanged a couple of emails with them at the time, but they don't intend to change it. I can't figure out why they would not let you add pictures, but they don't.
However...you CAN add text, so you could put the URL of another picture in the added text. You can edit text (I believe) up to 12 hours before the end of the auction.