In its pre-Instagram, pretelevision, all-but-pre-photography day, George Caleb Bingham’s “The Jolly Flatboatmen” (1846) was a viral image, a joyous genre painting of America’s westward expansion that became wildly popular through mezzotints and lithographs. The work itself, considered one of the most important American paintings of its kind, has hung in the National Gallery of Art in Washington for so many decades — regularly since 1956 — that it long ago came to seem like a part of the museum’s fabric.