A buckle looks like a wave. There can be one wave between two pinned points, or two or more. It takes more energy to form two waves than one so the one wave is a lower energy state. It requires some other constraint, such as a brace near the center node to get two waves.
To understand this, it is best to study the buckled shapes of simple beams. Probably the best resource in this topic is Timoshenko’s classic "Theory of elastic stability".
So short wavelength buckling would correspond to two waves between the pinned points, and long wavelength correspond to one wave between the pinned points?
Upper right could be a short wave, though typically is even short compared to structures length. Lower left is still a long wave. Why is this an issue?