Have you tried these instructions? If this doesn't do it I don't know what will.How do I replace an image in an active listing?
1. Locate the listing in your Saved Listings folder and click on its title to open it in the one-page lister.
2. Scroll to the Image section.
3. If you would like to take a few pictures out, click the Remove link under each picture you would like to remove from the listing.
4. If you would like to replace all the pictures or add pictures to the listing, click on any image to bring up the image selector pop-up window. To replace all the pictures, click the blue Reset button at the top of the pop-up. Then, click on the pictures you would like to appear in the listing. You should see numbers appear on each picture to tell you the order in which the pictures will appear on your listing. To add additional pictures to the listing, simply click on them. You should see numbers appear on each new picture indicating they are being added to the images that are already there.
5. When your image changes are complete, scroll to the bottom and click the Save button.
6. Return to the Saved Listings page and check the box next to your updated listing.
7. Click the HTML link on the right and copy the HTML in the popup box.
8. Locate the item on eBay and click the "Revise Item" button.
9. Click "Edit Description" then click the "Enter your own HTML" tab.
10. Paste the copied HTML in place of what's there and save your changes.
It's worth a try and not nearly as complicated as it sounds..
EDIT - But since relisting was done through Ebay, I'm pretty sure you will have to pay for extra pics.