No one suspects that a bunch of child art students could pull off such a complicated heist. So they carry their "blank" canvases out of the museum without anyone giving them a second look. Obviously, the stolen paintings are hidden under the blank canvas.
Something else was going down while Kat was pulling off her heist, though.
Leonardo da Vinci's http://www.leonardoda-vinci.org/ painting, Angel Returning to Heaven, was stolen, too. In its place was another calling card: Visily Romani. "Mr. Romani" strikes again.