Plex Automation

I’m trying to have my TV open Plex and go to a particular file. The library is called Workouts and has .jpg files in it. I have the Plex integration installed and trying to call this service from Developer Tools but I get “Failed to call service media_player.play_media. Unknown error”

service: media_player.play_media
  media_content_id: "'{ \"library_name\": \"Workouts\", \"title\": \"BDEPT Workout 01.jpg\" }'"
  media_content_type: photo
  entity_id: media_player.plex_exercise_room

What am I missing?