Set entity_picture: for custom sources

[craggyh]

Is there a way to set the entity_picture: attribute based on the source being played?
I'd like to do this to manually set cover art for HTTP radio stations when last.fm doesn't provide any.
This would be useful to see at a quick glance what online radio station is being played in the custom:mini-media-player card.

[nagyrobi]

See https://community.home-assistant.io/t/pass-by-url-the-entity-picture-attribute-of-a-media-player/324073