There will be instances wherein you have uploaded the photos to your listing but after saving, the thumbnail that appears on the listing is not the one you're looking to show
The system uses the first photo in the gallery as the thumbnail of each listing
If you're looking to change it, you would need to access the content from the gallery
Once you're in the Gallery section, click on re-arrange and this page should show up:
Once you have moved the photo, it should show up like this on the gallery:
Click on Save Changes.
It should now appear like so: