A picture that is not super tall, or a picture that's wider than it is tall! Whenever I next redesign mine, I definitely want to make it less tall than it is now.
This is just a thoughtful thing -- if your thank-you picture is really long, it takes up a bunch of space on the other person's wall, which can feel like more of a full-page ad than a thank-you.
Practically, it's a good idea for the important parts of the art to be away from the edges on the banner, since the website is a little bit responsive and will cut off the edges of the art depending on the size of the readers' browser window.
But for my banner art, the thing I was kinda thinking was, what would make me want to read this comic? And for me, it's the characters and how they interact with each other, so I made sure the banner showed that. (same thing for the thank-you -- which is a nice bonus because I always like the idea of the characters interacting with you a little bit as thanks. xD).
This isn't to say "I think character interaction makes the best banners/thank-yous" -- it's that since character interaction is what I love most, then if you also like character interaction you'll probably like my comic. If you feel like your favourite part of your comic is the designs of your characters & world, or the atmosphere of your comic, or whatever, then it might work even better to showcase those aspects!
