Find the answer to your question
It reads in 691 Trading API release note :
By late October, on sites with free gallery, if a listing uses a self-hosted picture (except in the case of a multi-variation listing), the picture will be copied to eBay Picture Services (EPS)
Q: If eBay copies a self-hosted picture to EPS, how can seller update that picture for te live item?
A: Seller can continue to do a regular ReviseItem and send in the new self-hosted picture URL. eBay will update the listing accordingly with the new URL
Q: If the old and new picture Urls are the same, will eBay reload the picture. In other words, will eBay reload the picture each time seller call RevisedItem?
A: We will not. We maintain the mapping between the seller’s external URL and our converted EPS image URL. We are able to identify if the image URL has changed or not in the request.