I had a similar problem with playing mp3 files with embedded thumbnails on my Sony Bravia TV. When the embedded image was larger than approx. 60 kByte, the Sony TV would ignore all tags (not just the image).
There might be a similar limit in your car mp3 player, but not necessarily the same limit.
You could try googling to see if others have figured out the problem, but you'll likely have to experiment yourself regarding what works.