Firebase Message without Icon

I have implemented firebase push notifications, in development mode everything worked very well. However, when signing the application, push notifications arrive but the icon does not appear.

For what I was finding out, it’s not a problem of Ionic or Firebase, the problem is the icon. Do you know the correct way to generate the icon? dimensions? etc etc? My icon is a transparent PNG, with a basic design. When running ionic cordova resources are generated correctly, but as I mentioned, for notifications is not correct.