My zenphoto (version 1.4.3.5) site using (slightly tweaked) zpBootstrap theme works fine. Many thanks to all contributors.

I have an issue defining (or setting) thumbnails for albums that only contain sub-albums. For a sub-album I can define a specific image that functions as thumbnail. That I can do for all sub-albums. So far so good. Now I want also to specify a thumbnail for the "one-level-higher" album in a similar way. There I am getting stuck and do no see any way to specify a thumbnail for albums that only contain sub-albums. Or is there a work-around?
Many thanks.

Admin -> Options -> Gallery -> Gallery behavior -> show subalbum thumbs.
This option enables you to pick a thumbnail from all sub-albums.

Many thanks fretzl. I simply overlooked the subalbum thumb option.