It happens when there are multiple images for a listing and we go through the process to send those images to eBay.  They get uploaded to eBay Picture Services (EPS) and then those EPS images are applied to the open listing instead of the ones from your own servers or the CA servers.  You're not allowed to mix and match those, but I've seen a few instances where eBay seems to get the image information corrupted and thus gets them mixed.  They should be able to self-correct through the revise listings process, but I've seen cases where they won't let us correct the images and so we get stuck with this error.

At the moment the only option is to end the open listing and create a new clean one.  I'm working on some changes that I hope will help allow us to be able to self-correct even more of these situations, but it's going to be at least a month before those changes are ready and even then there will be cases eBay does with the listings that we won't be able to correct since they won't let us apply the changes to correct them.  You could try manually editing the eBay listing and removing all of the images, then having the CA revise listings process go through and re-apply the images to the listing.  That might work, and then if it doesn't all you're out is a little bit of time and you still end up closing the open listing and submitting a new one.

We've made some changes here that went live at the beginning of the month which I would have expected to address many of these.  If you're still having them please raise an issue with support so we can look into them and see if there are further enhancements we can do to prevent them from coming up in the first place.

The message comes from a conflict eBay has when there are multiple images on a listing at the top left -- if there's only one they don't have any issues, but if there are more than one then they all have to be eBay-hosted images.  We automatically send your images to eBay so they're eBay-hosted, and make sure that we update the entire listing to be all eBay-hosted images.  If there are some listings running into this error that means they're trying to create a conflict situation between eBay-hosted and self-hosted, so we need to understand what's happening and how we can adjust to make it be successful.