Thanks Kirk!
I also discovered that when I set the preview zoom to the upper half of a portrait image this zoom setting is kept when advancing to next or previous images but is lost if I exit and then reenter the preview again or even if a landscape image comes after the portrait ones.
I don't know again if it is bug or feature but since most of my images are portrait, I would feel more comfortable to have PM remember which part I zoomed both portrait and landscape images (not diferentially for all images of course, but the last one of both kinds basically).