Edvard Munch’s portrait of existential angst is the second most famous image in art history – but why? Alastair Sooke tells its story. Beneath a boiling sky, aflame with yellow, orange and red, an androgynous figure stands upon a bridge. Wearing a sinuous blue coat, which appears to flow, surreally, into a torrent of aqua, indigo and ultramarine behind him, he holds up two elongated hands on eithe