I like to have rectangular thumbnails. With the former versions (1.2.6) of Zenphoto/Zenpage, I had a rectangular "cropping pattern" and not a square one in the CMS.

Is there anything I could change to get a rectangular pattern?

Options > Theme > Crop thumbnails. Those are percent values (and always where) dependend on the actual image size as image don't have the samesize actually.

Another way is to modify the theme to use custom image functions.