Painting one portrait of Barack Obama for each day of his tenure as president of the United States—for a total of 2,922 pictures by the time Obama leaves office—Rob Pruitt is making a “monument . . . about the entire presidency.” Each day, Pruitt browses online news sites in search of images of the president, and then, when he has settled on one picture, loosely brushes its contours with white paint onto a ready-made two-foot-square canvas primed with bands of red and blue pigment. The Museum of Contemporary Art hung as many of the paintings as its gallery walls could reasonably