Album Flow will bring back the Cover Flow feature for your Mac. In iTunes 11, the Cover Flow view is not available anymore, so if you like that view, then this is the perfect app for you.
Album Flow for iOS has been released more than two years ago, and this app is almost the same for your Mac.
If you are curious about the artist what you are listening to then look to the right side of the screen. You can see the complete discography from Last.fm.
Missing album artwork download
If there is no album artwork in iTunes, then the application downloads the album cover from the internet, based on the album title and artist. So if you have the correct artist and album name in your music information, than there will be no more "no cover" image.
If you have any question or suggestion regarding the application, please don't hesitate to drop a line to firstname.lastname@example.org