These pictures make me realize once again how friggin' huge the space shuttle really is! Without a point of reference it looks rather small, because the cockpit windows are so big. But seeing it strapped to the back of a 747 really puts the dimensions of the shuttle into perspective.
FSXF-Psycho: The museum is not going to clean Discovery any further. They decided that since she flew 39 missions and spent about 365 days in space, leaving her weathered and used is the appropriate look.
I would also imagine that all that discoloration on the fuselage is burned in for good from the tremendous heat during reentry. In that case "cleaning" would mean to replace the panels with brand new ones, which kinda reminds of the old joke: "Yes, this is George Washington's axe. The handle and the head were replaced a few times, though".