This guide shows how to put a specific thumbnail to a on particular livestream
You can have a default thumbnail for your livestream and still attache another specialized for one specific event or livestream to make it stand out. By doing following:
Enter the game or live event for which you want to apply the specific thumbnail.
Press Advanced Settings
Then press Thumbnail
Then press Upload new thumbnail , and choose your photo, then save when uploaded.
Note that this thumbnail will only appear on this particular livestream.