A playlist is just a collection of tracks, so the tumbnail view for a playlist has a thumbnail for each track (no album art option for a playlist view).
If you want to show one thumbnail per album and keep things separated and chooseable by album, create a playlist-limited library view scheme like this:
1. Create a playlist called "favorites" and add your favorite albums to it.
2. Create a new view scheme under the library (call it favorites). Limit it to the "favorites" playlist. (That will be under item 4 at the bottom in the create view scheme dialog window, click on the down arrow, playlist and "favorites")
3. You can have whatever panes you want in the view scheme, for instance you could have genre - artist- album or year - album or just album, in any case it will only show the albums in your "favorites" playlist.
Then below the panes, right click and choose "customie current view" and choose "album thumbnails" and that will show you a thumbnail for each album.
Have fun!