Three images control how your channel looks when you are not live and how it looks when someone shares it. All three are in Appearance.
Shown on your channel when you are not streaming. It is scaled to the viewer's screen and displayed at 16:9, so design for that aspect ratio rather than assuming a fixed size.
A premium feature. It shows above the chat panel on your channel; 500 by 80 pixels is the recommended size. You can make it a link, so a banner advertising commissions or a Patreon can go straight there.
Used when your channel is linked somewhere else: Discord, Twitter, Tumblr and anything else that renders a preview of links.
Those services cache the image: if the old one still shows after a change, you likely just need to wait a bit for the cache to clear.