Hey Matt!
I believe you can find what you are looking for here: https://help.twitch.tv/customer/en/portal/articles/2348988-adding-a-game-and-box-art-to-the-directory
Specifically, the last paragraph on that page which references “Box Art” and provides a link to a Google Docs form 
This is to gain access to the Twitch Developer portal which allows you the update/manage box art.
Hopefully this is the information you were looking for