How do you add thumbnails to posts?

I'm tryin to see if you can add a picture as a thumbnail in a thread without havin to make it the first thing ya see in a thread, if at all. I see some people pull it off but idk how help pls ;-;

So for the sake of argument we have a discord emote in the link i wanna use. HOW DO IT?

EDIT: thanks guys you rock >:D