An old thread of mine, but I thought I'd post a reply in case anyone else is suffering from this. After contacting tune-in radio via their email address and also a few of the radio stations affected, they both said the issue wasn't at their end.
I then started to look at what may have caused it and the only other thing was Android Auto app itself. It seems from the history that it upgraded itself from Version 4.9.594934 to 4.9.594938 back on December 12th (about when the issues started). I managed to uninstall AA and then reinstall the last known working version (4.9.594934) and bingo, everything went back to working with all the track album art showing across all stations I listen to.
For the time being I've disabled auto update on my phone to ensure it doesn't suddenly reinstall the non working version again, but hopefully in the near future AA will fix the issue and issue a new update.