F&W! You add pictures by clicking the "edit" button, then clicking on "add photo", selecting the picture to add, then uploading it. Just the same way that you added the original pictures to the auction.
If I understand correctly, you will only ship on the 26th, which is the 4th Wednesday of March, even though this auction ends on the 13th. Maybe you don't know since you're fairly new, but Listia rules require sellers to ship within 7 days of the auction end date, which is March 20th in this case. Just FYI.