I just tested again and I was able to revise an active listing from Auctiva, revising fields, quantity, and adding images from the Auctiva Active Listings page using the icons on the right hand side of the listing. The images I added were appended to the bottom of the description, and eBay appends the text "on <DATE>, seller added the following information:" followed by the images I added.
If you are specifically referring to the "eBay Gallery Images" (the images that show to the left of the price in the listing, not the images in your description), those images are eBay images, stored on eBay servers. You can tell this by inspecting the page and reviewing the image URL where you will see something like "http://i.ebayimg.com/images/ ..." If you use Auctiva to create listings, then the images showing in the eBay "gallery images" would have been uploaded to eBay where eBay then makes a local copy to use in the listing. Within Auctiva, we do not currently have a revise tool for the eBay Gallery Image (partially because it is an eBay image), but you can always use our create listing from similar to change the eBay gallery images. If you are revising this listing on eBay, I can't say for certain why it's giving an error.