Sorry, I can't find a way to do this. While looking at an album, I see that some photos need to be rotated or flipped (even after auto-rotate gets most of them right!)

How can I enable quick edits (rotat 90, 180. or 270 degrees) while browsing photos?

on the admin image tab under "utilities" (on the right side.) But maybe your graphics library does not support image rotation, then this will not be available.

Zenphoto does not support any "editing" from the front end as it is terribly insecure to allow.