From left, artists Dave McKenzie, Mickalene Thomas, Julie Mehretu, and Theaster Gates.   DAVE MCKENZIE, MICKALENE THOMAS, JULIE MEHRETU, Njideka Akunyili and Theaster Gates are in good company. The artists are among 13 nominated for the biennial James Dicke Contemporary Art Prize. The Smithsonian American Art Museum announced the nominees on Aug. 14. Established by...