you have to do this via my eBay or equivalent. Go to 'revise your item' then into the description area, from where you should see a further link to 'edit your picture'or something like that (talking off the top of my head here) If no-one has bid on your item, you can amend your pictures for the first 12 hours of your auction, but after that, I am afraid you can't.
Oh, and by the way, today I couldn't get into Auctiva for the first 3 hours of the day, and now I am in and halellujah, my pictures are showing. I wonder how long for?