Why the images will not update/revise
What happens is when you don't upload your pictures to eBay, and use your own self hosted picture, we take that picture and automatically upload it for you, then return an eBay picture URL and use that. But we keep the URL of the original image in our database. If you sent us an invalid image, or update your self hosted image, and then try to revise the item with the same image, eBay will treat it as same as what you originally sent. And no update will be made as we assume you are using the same image.
Workaround - So now what you need to do is get the picture we have stored in our database changed. What you can do is trick it by supplying a different image name, or appending a cache-busting parameter to the URL in the revise request, such as appending a "?v=2" or "&v=2" (use this if the URL is parameterized), which will result in the item data being updated.
UploadSiteHostedPicture - http://developer.ebay.com/DevZone/XML/docs/Reference/ebay/UploadSiteHostedPictures.html
Answer Title: Image Hosting: EPS Vs Self-Hosting
Answer Link: https://ebaydts.com/eBayKBDetails?KBid=1505