Nothing fancy, but varying. For local files I’m currently using Lollypop on my Linux desktop and Metro on my Android. But actually I use my Funkwhale server a lot, especially on mobile.
I personally don’t need more than a nice display of the front cover, so don’t really care about the back cover or any other image for my local files.
I really looking for something for the lounge on a big ish TV… what itunesLP promised… maybe Kodi or Logitech Media Server… of course without the appropriate good artwork, it won’t do anything…