Hello i'm using the youtube api insert method https://developers.google.com/youtube/v3/live/docs/liveBroadcasts/insert to post a new broadcast to Youtube but i want to change the default image youtube is inserting on the broadcast here is the image youtube is giving by default

When i change the 'thumbnail' property on the livebroadcast it does not work can somebody please tell me how? and what's the difference between slate and thumbnail please?