If your eBay gallery image is not showing on eBay, the best thing to do is to revise the image so the gallery image will re-generate.
The one issue here is that if the gallery image was sent to eBay originally then you will need to revise the image to a different image first.
The reason for this is that if there is no change, then eBay will not consider it a revision and will leave the image alone.

So, to do, you need to go to the Marketplaces > eBay > Open listings view.
Select the ad(s) you want to revise and hit the "Revise listings" option. (Multiple listings can be revised at once)
For the image url, select the check box and then enter in the image url of the image you are revising to.
It would be advisable to use a stock image or your company logo for this part.
Hit save and exit and then check the open listing to see if the image revised.

If so, repeat the previous steps but this time enter the actual image you want to use, template tags are also fine to use here.
For example, you can enter {{IMAGE(ITEMIMAGEURL1)}}.

The listing should then show the correct image, and the gallery image will also be updated.