My car stereo is a newer model Pioneer AVH-501EX.
It requires 600x600 pixel music file art work.
When I use Mp3tag > Tag Sources > Cover Art > Discogs to get Art Work, it does not show up on my car radio.
When I extract the same image using Mp3tag, re-save the same image without making any resolution changes or any changes to it - other than re-saving it as a JPG file and then manually Add cover... using Mp3tag... then everything is fine.
This leads me to conclude that something about applying the Art work directly from Mp3tag > Tag Sources > Cover Art > is a problem. Applying it manually through Mp3tag > Add cover... is not a problem.
If anyone else ever mentions this, maybe keep my report in mind.