President Barack Obama has taken a decidedly low-key approach to racial issues since he became America’s first black president two years ago. But in a hallway outside the Oval Office, he has placed a head-turning painting depicting one of the ugliest racial episodes in U.S. history. Norman Rockwell’s “The Problem We All Live With,” installed in the White House last month, shows U.S. marshals escorting Ruby Bridges, a 6-year-old African-American girl, into a New Orleans elementary school in 1960 as court-ordered integration met with an angry and defiant response from the white community. The thrust of the painting is not...