Mastodon tip...
If you just include the youtube link in your toot, it will automatically grab the thumbnail and show it in the toot.
(But if want different thumbnail to be in your toot, you can do it as you did and put up an image separately.)
Welcome to qoto.
@Pat thank you - indeed, i hunted that specific shot of the trees, the thumbnail YouTube chose wasn't that. I love that specific shot. Thank you for the tip.