Washington Crossing the Delaware, the city’s most popular painting, belongs to its largest museum. But for a while, in the last century, we didn’t quite know what to do with it. An old-fashioned behemoth, 21′ x 12′, it seemed plain as a sermon. Perhaps it was kitsch. It got shuffled to Washington Crossing State Park, the site of the event it depicts; then, after it came home, it hung in a narrow gallery where you could not stand back from it properly to view it whole. But one of the benefits of postmodernism is that we may look at paintings …