Entity_picture png-file circular cropping

Configuration Frontend
1

I have a motorized skylight. I set it up like a roller shutter. This works fine. But I would like to use a different picture for this. In the customize.yaml I have set up this for the entity of the ‘Cover’:

cover.bre_et148_cover_1:
  templates:
    entity_picture: >
      if (state == 'closed') return '/local/png/fts_light_dome.png';
      return '/local/png/fts_light_dome_open.png'

This is what the PNG should look like:
fts_light_dome fts_light_dome_open

But this is shown in the GUI:
Luke

The PNG are obviously cropped in a circular shape.

Who knows why? How can we prevent this?

Thanks in advance

Bernd