When you go to post the item, soon as you hit “Next” after selecting the item your selling… hit F5 on your keyboard (if on windows) to refresh the page, this will make the default photo picture at the top not load and you can add your own photo into the spot where the default photo would be.
If you refresh and it brings you back to the items list where you select which item you want to sell, just go to your listing’s as it will be there and you can edit the post and add your photo. Hope this helps!
Im posting this because for some reason we cant add custom thumbnails for MOST items (no idea why) and this is how you can 