How to assign entity picture, for use with tile card for example

I’m trying to use the show_entity_picture in the tile card, and can’t quite figure out how to make it work. Please correct me if I’m misunderstanding, but it’s a simple way to show a local image instead of the icon in ha-icon, right?

I’m wondering how I assign the actual entity picture, and how I can then test if the assignment “took”, because I’m not seeing it on the card.

I have this in my configuration:

homeassistant:
  customize:
    cover.gluggatjold_bordstofa:
      entity_picture: /local/png/dining_room.png

Turns out I had this ALMOST right, just had to change entity_picture to picture in the configuration. Leaving this for posterity.