I know over the years, I have seen many posts regarding missing cover art on car players.

I've just added a Pioneer AVH-3100DAB to my car and had a play to see which art was and wasn't displayed.

On the Pioneer AVH-3100DAB, the art must be embedded, though you can also have embedded and Folder.jpg or cover.jpg.

Full-size art was displayed from 100x100 with a very pixelated display of 6KB, up to and including 600x600 with a file size of just over 100KB.