Picture a nude George Washington riding a white horse, wearing high heels and nothing but a coquettish pout on his lips. Now imagine a portrait of the scene hanging at the National Portrait Gallery in the nation's capital. That's not much different from what is happening in Mexico's most prestigious museum, Palacio de Bellas Artes, which is displaying a controversial portrait of Emiliano Zapata, one of the country's most beloved revolutionary heroes. The small painting by 32-year-old artist Fabián Cháirez, called "La Revolución," has provoked outrage and protests by a handful of Zapata's descendants and a farmers union. They are...