Here are some instructions, courtesy of Auctiva Mike...(these have gotten me out of a jam more than once)
Hi Community,
Instructions to replace an image in an active listing:
1) Log in to Auctiva and click the "Images" tab.
2) Check the box next to the image you'd like to replace in your active listing and click the "Replace" button.
3) Select or upload an image to replace the checked image. This new image will assume the same URL as the one it replaced and be visible in your listing.
You can change the images, or anything else within the Auctiva template, of an active listing by revising it via the following process:
1) Locate your listing in your Saved Listings folder, make any necessary changes, and save.
2)Check the box next to your updated listing in your Saved Listings folder and click the “Get html” button.
3) Copy the html in the popup box.
4) Locate the item on eBay and click the "Revise Item" button.
5) Click "Edit Description" then click the "Enter your own html" tab.
6) Paste the copied html in place of what's there and save your changes.