"Car for sale" thumbnail not changing

I understand (I think) that the thumbnail for the post is the first photo. I've changed the photo out a few times, but this doesn't seem to work. I've checked the posts here and tried those tricks, to no avail.

How can I change the thumbnail of my post (so it's the actual car and not the box of parts)?

Thank you.

 

Try deleting all your pictures and reinserting them. I’d suggest starting with only one picture and check to see if it changed on your thumbnail ad.
Then add the other pictures afterward.

 

Thank you.