Manage Listing - Change an Item's Picture

In order to change the picture that goes to your item description  - if you are in the Auto lister, simply choose the desired picture from the PHOTOS area (click the small black radio button) and continue listing - you would only see the change after the listing is listed, as the auto lister doesn't reload data in the middle of working on a listing. Basically, the idea is that the main gallery picture is the one that is eventually taken to the description:

If the listing already exists, please open item editing window (click the pencil icon on the right side of item row in MONITOR page):
 

Then choose the picture you want it to become (it would also become the main picture in the item Gallery in eBay) and save:

Editing an existing listing currently only supports non-variation items.