This spring, hundreds of thousands of visitors to Mexico City’s Museum of Modern Art experienced the ardor of two of Frida Kahlo’s masterpieces in a temporary exhibit: “Diego on My Mind,” depicting Kahlo with Diego Rivera’s portrait emblazoned on her forehead like a third eye, and “Self-Portrait with Monkeys,” showing her surrounded by four monkeys amid dense tropical foliage. (Rob Farmer/The New York Times)