The problem is one I came across a while back.
When you add a group of images to a listing via Auctiva they are all held within the description details.
If you add further via ebay -> revise item they will charge extra as they are added external to the description detail as they would be if you had included multiple images via ebay in the first place.
The workaround is either to tweak the HTML via ebay -> revise item or often safer to first simply revise your saved listing on Auctiva.
Then copy all resulting HTML for that listing via the saved folder entry for that listing and paste it in the live listing's description HTML window via ebay -> revise item.
Preview before committing/submitting in case something got damaged, then you will not be charged extra image fees. This is all perfectly legitimate.