Boo's Portrait

From the Super Mario Wiki

Boo's Portrait is an item used by Mario in Paper Mario to access the third floor of Boo's Mansion. It is a living picture of a Boo with a top hat and monocle, but when Mario first sees it is a blank picture that asks Mario to find its body. Mario found it in the mansion's library, on top of a bookcase. After finding the portrait and putting in its frame, the portrait lets Mario jump inside it (a la Super Mario 64) to warp to the third floor of the mansion.