This picture brings across the message of friendship and something even deeper than that with the trio starting together and ending together. The idea is that even when something indestructible (Hogwarts) falls, their friendship will still stand.
“If I had a world of my own, everything would be nonsense. Nothing would be what it is, because everything would be what it isn't. And contrary wise, what is, it wouldn't be. And what it wouldn't be, it would. You see?”