Tom Tomorrow helpfully provides an artist's conception of "the news photograph they didn't want you to see."
So I'm sure you've heard about the Picasso anti-war painting "Guernica" that they covered up at the UN building already. "Mr. Powell can't very well seduce the world into bombing Iraq surrounded on camera by shrieking and mutilated women, men, children, bulls and horses."
But here's a fun bit of trivia I hadn't heard before: the painting was donated to the UN by Nelson Rockefeller, and...
Nelson Rockefeller himself started the tradition of covering up art donated by Nelson Rockefeller when he sandblasted Diego Rivera's mural in the RCA Building in 1933 because it included a portrait of Lenin. (Rivera later took his revenge, reproducing the mural for display in Mexico City, but adding to it a portrait of John D. Rockefeller Jr. drinking a martini with a group of "painted ladies.")