Just a couple months ago, Android Auto added six preset images hidden away in the code that could be used as future backgrounds. We had been hoping that this meant that full custom wallpaper support would be coming soon, but instead, Google has removed most of the previous images in favor of 12 new, more car-oriented backgrounds that suit Android Auto a lot better. They're still not selectable, though.
These backgrounds, pulled from the Android Auto 6.0 update today by 9to5Google, are actually pretty pleasing to the eye. There are a few with leather stitching for those who value luxury, a few performance-oriented ones (a drilled rotor, a carbon fiber pattern, and a close-up of a grille), some sound wave-related ones, and a few landscapes with light trails as well. To my eyes, these new backdrops seem a lot more suited to an in-car application. Whereas the previous ones just seemed like the same type of wallpapers that Google would load onto a phone or tablet, the new ones definitely were hand-picked for Android Auto.
These backgrounds aren't live just yet, but they were found in the Android Auto 6.0 APK. If you haven't yet received the Android Auto 6.0 update on your phone, don't despair; we've got the APK over at APK Mirror. Personally, I'd be using the brake rotor close-up as my background... if I still had a car with Android Auto.