Image thumbnails for a LiveBroadcast object

I can't seem to associate image thumbnails with a LiveBroadcast object as per the documentation when trying to create a live stream via the API.

For my application, I am using the following reference:

https://developers.google.com/youtube/v3/live/docs/liveBroadcasts#snippet.thumbnails

Here's an example of my payload sent to /liveBroadcasts:

array:3 ["snippet" => array:5 ["title" => "Test broadcast""description" => "Test description","thumbnails" => array:1 ["default" => array:3 ["url" => "my-image""width" => 120"height" => 90      ]    ]"scheduledStartTime" => "2021-03-01T21:35:37.000000Z""scheduledEndTime" => "2021-03-01T21:40:37.000000Z"  ]"contentDetails" => array:2 ["enableAutoStart" => true"enableAutoStop" => true  ]"status" => array:1 ["privacyStatus" => "public"  ]]

The URL to my-image of course is actually a working URL.

Must the images in thumbnails be previously uploaded to YouTube and hosted there instead? What else could I be missing?

Thank you!