Thanks for contributing to our forum.
I just had a chance to do some testing with respect to this matter and, unfortunately, the only way I ended up being able to successfully change one variation image in an active listing was to remove all of the variation images and then reupload all of them using one method.
The error message suggests that you should be able to replace individual images by using the “self hosted” option, which is effectively how they are submitted from Auctiva, but I was unable to find the option to replace these images using the “self hosted” option on the interface.
I regret that I cannot offer you a better solution, but I hope you are at least able to proceed now. If you need any more specific assistance with this, please contact our Customer Support team directly from under the “Help” tab on our site and we will be happy to continue working with you.