This is a great area to capitalize on for branding or even providing information to your viewers in the form of a schedule, social media, and any other info your viewers might find interesting.
Notice that when you are hosting someone your Offline image won’t be visible.
Navigate to your Twitch channel settings page https://www.twitch.tv/settings/channel
On the Menu on the left side click on Preferences and then on Channels. Scroll down till you see Video Player Banner settings