Android image only push notifications

Question

Non-expanded push notification on lock screen (same as notification bar) - OnePlus 7 Pro

Expanded push notification in notification bar - OnePlus 7 Pro

What's the API that's used for achieving this? I've been looking around in the firebase documentation as well as browsing the android API and couldn't find an example. Other notifications look how you'd expect them.

Couple of notes:

• I noticed that right when I receive this notification, it has text and body, and after the image loads, the text and body are no longer visible.
• The notification has two images, one for non-expanded view, and another for expanded view. It's not the same image, it's just fading between the two.
• The app is Rest Poker on play store if you wanna give it a try.

I currently use the firebase FCM API, but I understand this might not be possible with it, but maybe by registering the notification on the device might do the trick, as long as we know how to set it up like this.