Although this is not really Linux related, I will document this anyway because this has cost me a couple of hours of my life.
The Audi MMI system is capable of displaying your Cover Art in MP3 files but at doing so it is very picky. So here are a couple of hints on how to get your Audi MMI to display your cover art.
Only embedded files are supported, no images in the same directory (you can use
EasyTag or
PuddleTag for this, both apt-gettable).
Only JPG is supported, no PNG. The JPG may not be progressive JPG (which is default in GIMP exports and also the reason why I couldn't find the problem at first)!
A maximum image size of 500×500 pixels (5“ display) or 700×700 pixels (7” display) is supported.
A maximum file size of 100 kB (5“ display) or 250 kB (7” display) is supported.
It doesn't matter if the image is defined as “front cover” or “other”.