For both the phone and tablet sizes it might make sense to either crop or scale down images that would appear larger than the screen size. Scaling down would leave black bars, but cropping might give a wrong impression.
^ uses 3 vertical columns in portrait, 3 horizontal rows in landscape; scrolls in opposite direction
The theory on this is that we'd do some kind of size adjustment like justified text to keep from having a ragged edge where the columns/rows wrap. Imagine that images are squished or stretched or given extra whitespace or something to fit.
Note I've included a couple extra images not in the previous mockup version; the first in the batch is now an "extra tall" image, to compare with the "extra wide" horizontal panorama.