I don't put that many albums on Facebook, but whenever I do I spend at least five minutes trying to remember how to change the album picture that appears on the timeline when the album is published.
The problem is there are many edit menus associated with Facebook photos. There are menus that appear on the photo, menus for photo descriptions, menus for editing albums and so on. The one you want is the edit menu associated with the timeline item, not with the album or the photo:
... To switch it with another photo, click at the pencil icon and select ‘Change Primary Photo’. You can then use ‘Reposition Photo’ if your selected picture doesn’t fit in properly."...
Now I might remember.