"Pablo Picasso's Guernica is so familiar, so large, so present. It's physically bigger than a movie screen. But what is the painting about? Is it an account of the Spanish town obliterated by Nazi warplanes - a piece of reportage? Is that why it's in black and white?

This is the reason why the painting has such an impact. Instead of a laboured literal commentary on German warplanes, Basque civilians and incendiary bombs, Picasso connects with our worst nightmares. He's saying here's where the world's horror comes from; the dark pit of our psyche."