You change pictures in the Image library, not on the Saved Listings page. Saved Listing are those you are preping/saving for future listing. You can only upload new images from the lister (accessed from Saved Listings), not replace old ones. Any old image including those pulled back from eBay, need to be Replaced using that feature in Manage Images. Note, the ones pulled back from eBay should show in the Imported Images Folder.
If Auctiva Imported the listing info from an Active listing, the pictures should show in your Images library in the Imported Images folder. It might be one of the few exceptions to active pictures captured (once after direct listing) and synced back, so they can populate the Store and SG. I believe they only do the pictures once, so updating them at eBay after the fact requires manual change at Auctiva.
Now that's from my notes..... However, they are constantly making changes on both systems, and I keep getting surprises as a result.