Deck Links That Show the Deck
Link previews show your opening slide. Paste a deck's share link into Slack or an email and the preview is a picture of its opening slide. It's made when you create the deck's public link. Refresh it from the Share window or from the deck's card menu.
Deck cards show the same picture. Deck cards on your dashboard use that image. It updates shortly after you change the first slide and your edit saves. Apps that already previewed a link may keep showing their saved copy.