Two paintings by Diego Rivera, including his 1915 Cubist masterpiece, have joined “Picasso & Rivera: Conversations Across Time,” an exhibition at the Los Angeles County Museum of Art that opened in December. “Zapatista Landscape” and “Flowered Canoe” had been on loan to the Grand Palais in Paris for “Mxico 1900-1950: Diego Rivera, Frida Kahlo, Jos Clemente Orozco and the Avant-Garde.”