I got this to work fine for images in albums (Thanks!) - but I can't figure out how to make it work for the preview image of an album. Is it possible to do in a template? isLandscape() does not seem to work for album thumbs.

Right that does not work with album thumbs. If you use the nightly build there are now new functions for this (currently undocumented!):
getCustomAlbumThumbMaxSpace() and printCustomAlbumThumbMaxSpace() (only for album thumbs) as well as
getCustomSizedImageMaxSpace() and printCustomSizedImageMaxSpace() (for both the thumbas within an album and the sized image).
See template-function.php directly for parameters

Nice! I'll have to try that out. Also looks much cleaner than this 'workaround' for image thumbs. I understand maybe not having this option in the zenphoto GUI because the CSS in templates is not built for it, but for those of us rolling our own themes, a function is a good step forward.

@ acrylian

How would I implement those functions? And where?

I'm on version 1.2.8 [4881]

My client's site is for fine art and nature photography. He doesn't like that his photos are getting cropped.. and not showing the full proportion. If I turn cropping off it's a mess because he has horizontal and vertical photos all over the place.

You really should read our theming tutorial first to learn how Zenphoto themes work. You will of course also have to adjust the theme's CSS.