Can't get image_state to work

Configuration
1

I’ve built a small card for showing a clock and date on my wallpanel.
I also want to show an image based on a state of a sensor.
The images are all PNG with transparent backgrounds
But I don’t see it update the image, what did I do wrong?

elements:
  - type: image
    entity: calendar.testing
    state_image:
      'off': /local/images/clock_background.png
      'on': /local/images/clock_background.png
    image: /local/images/empty-clock.png
    tap_action: none
  - entity: sensor.time
    hold_action: none
    style:
      color: white
      font-size: 5vw
      left: 60%
      top: 35%
    tap_action: none
    type: state-label
  - entity: sensor.datedk
    hold_action: none
    style:
      color: white
      font-size: 2vw
      left: 60%
      top: 78%
    tap_action: none
    type: state-label
hold_action: none
image: /local/images/empty-clock.png
tap_action: none
type: picture-elements