The problem occurs if eBay cannot fetch the image for one of the reasons:

- image is only available via HTTPS protocol on your site;

- the URL on localhost/development host is not available outside network;

- due to some network issues;

- .htaccess settings which make access to images forbidden.

On the other hand, it might be a problem with eBay service. For example, if eBay is overloaded with the requests to its image hosting server, it may refuse some requests.

As a solution, you need to make sure your images meet all the requirements. If they do, please wait a bit and try to revise a product anew.