There is a way to force the album art to be displayed in the lower left manually. Go to the Legacy tag window and select the small thumbnail in the upper left - that will also place the cover art at the bottom of the tree (close the tag window to see). Then go back to the Legacy tag window settings to revert to the Modern tag window - the album cover will still be there when you close the tag window. Unfortunately, this is not remembered between MC restarts, so you have to go through this multiple step procedure to manually show the album cover each time.
If your reading this Matt, any chance it is easy to make that something MC remembers between sessions? It is definitely something I always want to have turned on, but every time I restart MC I have to do the tag window dance all over again. Another approach might be to add it as one of the action window menu choices: Rip Disc, Burn Disc, Sync Handheld, Camera, Build Playlist, Tag, and now... File Image