It’s the job of MP3 players to either display rectangular image with white borders or to pan and scan (zoom and crop) it to square.
We should not upload cut off covers for this or that purpose, it must all be post edit by the specific needs users. 
Maybe it could be a Picard feature to auto squarify front covers for the people who want it?
Auto‐square front cover (zoom and crop)
Update (2017-08-02):