The Zenpage theme shows subalbums first, then the images within the current album. I want to reverse the order. How could I do that?

Youn could switch the next_album and next_image loop. But this is not really a theme matter but a core order (since albums are "above" images level wise) regarding pagination and wll not work well.

We recommend to use either just (sub)albums or images within an album which is also better for visitor usabilitty.