and that's one more reason spotify is better than pandora. You can change the cover image of any playlist or station to anything you want. There's a reason spotify is rated the number one streaming music service on any site you go to. Ditch pandora and go with spotify. Use a music locker service to transfer all your stations and playlists over and never look back.
you can work around it for now. Delete the station that you want as your cover then + Add Variety it back in. what ever is at the top of your add variety will show up as your station icon. I had to do this a couple of times because I don't understand how they decide to put which song or album where in the list but keep at it and you can get the picture you want.
Sometimes the images get "stuck". I went into my "Add variety" section and deleted the song that had the album cover that had become the permanent thumbnail for that station. Then I went and added it right back in. Now the station thumbnail reflects whatever is playing on the thumbnail.