When scheduling a live broadcast on Facebook or YouTube, a preview image can be provided. On Facebook, this will be used for the announcement post. On YouTube, this will be displayed as a video thumbnail.
Image Size and Shape
When creating an image, use a 16x9 shape. For example:
1280 pixels wide by 720 pixels high
1920 pixels wide by 1080 pixels high
Other shapes will be cropped by Facebook.