“The arts editor had a brilliant idea. He thought. A weekly series on portraits? I wasn’t so sure. The National Portrait Gallery is my idea of hell. Hard-faced Tudors and luxuriantly eccentric Victorians are great. But the 20th-century galleries make your flesh crawl with their bad paintings, trite photographs, and affirmation that the interest of a portrait lies in its subject rather than creator. The NPG classifies portraits by the person, with the artist’s name second. This is why portraits can seem the opposite of serious art.” And yet…