Nusch Eluard was the wife of Paul Eluard, a French surrealist poet. She was an actress, acrobat and close friend of Picasso whom he painted many times. “Portrait of Nusch Eluard” (1938) is the colour of smoke, composed of flat, interlocking planes of elusive grey. She clutches her handbag tightly. You sense the neat wool of her suit. Three years later he paints her again in “Madame Paul Eluard” and it’s very different. The same skinny ribbon ties her hair but she is a pallid boy-girl, her already thin body shrunken further by wartime deprivation. The paint is soft and smeary, no crisp outlines now. She is downcast, dissolving, her eyelashes damp slashes in her face.

Picasso’s wives and lovers mark the phases and shifts of his life. Jacqueline Roque, his second wife, appears in many guises: in one painting she wears a black headscarf like a Spanish matron; in another she is cut out of bent sheet-metal like a heroic female tail-fin. In “Claude drawing, Françoise and Paloma” (1954), his long-term lover Françoise Gilot is the matriarch. It is a sombre painting, terse, concentrated and unsentimental (the couple had separated in 1953). Their children are tectonic plates of green and blue to be moved around and the scored outline of their mother forms the architecture that holds them. The shadow of Claude’s pen on the page is like a knife.

​Picasso puts himself through the wringer too. One of the first paintings you see is “Self Portrait with Palette” from 1906. He is thick set and thickly painted, confident, his hair jet-black and smooth, his face blank and ready for anything. At the other end of his life, in “Self Portrait” from 1972, the readiness is long gone: he is a chalked gargoyle, eyes unfocused and blank. His head balances (just) like a stone atop a cairn.

As much as he painted portraits, he painted portraits of paintings. He loved to set himself against other artists – Velázquez, Goya, Rembrandt, Cezanne – and his dialogue with the art of the past never stopped. He takes on Velázquez’s “Las Meninas” in shocking red blocks; “Jaume Sabartés with Ruff and Cap” (1939) is an El Greco steal. It’s a beguiling picture of the man who was Picasso’s long-term factotum. His nose goes one way and his glasses skew another. His eyes are little criss-crosses and his mouth is pursed. It’s prissy, funny, affectionate and knowing.

​Like many of these pictures, it is imbued with the spirit of the cartoon. In 1957 Picasso said, “There are so many realities that in trying to render all of them visible, one ends up in the dark. That is why, when one paints a portrait there comes a moment when one ought to stop, having attained a sort of caricature. Otherwise, at the end, there would be nothing at all.”

But the comedy is shot through with violence. “Old Man Seated” (1970-71) was painted when he was 90 and it is bonkers. Picasso did not go gently on old men. In orange and green curly-wurly paint, he fuses the figure with the armchair, white stomach bulging. And there is an astonishing, rarely seen portrait of the Parisian alcoholic “Bibi la Purée”, a grinning, wet-lipped drunk with a degraded face like an old clown. He had only a bit-part in the Picasso story yet the painting comes like a slap: a defiant figure at the end of his life brought out of the dark and placed centre-stage.