Each media can have it's own poster image. For videos, they allow the video element to be hidden with an image, so that the video underneath doesn't show until it has loaded or been played. The poster image can be set to only show at the beginning, or show any time the video is paused.

Poster images are also useful for the audio version of the player, for displaying the art for each audio track. It can be displayed anywhere inside of the UI via the control option.