By previously adjusting the canvas size of the landscape photo, so that it has the same relative proportions as the portrait orientation, the full photo is shown. Normally, the empty canvas space would be filled with white, but here I have filled it with a colour exactly matching that used in the background of the chosen Sandvox design.
Yes, the landscape oriented photo looks small, but the canvas can be stretched in all ways: a square canvas could be chosen, for example, but that would mean changing the canvas sizes of both landscape and portrait oriented photos.