From the Super Mario Wiki, the Mario encyclopedia
The portrait itself is a living picture of a Boo with a top hat, monocle, and a moustache like Bootler's. However, when Mario first discovers the portrait inside the mansion, it is only a blank, talking picture frame that asks Mario to find its missing body. Mario finds it in the mansion's library on top of a bookcase. After finding the portrait and putting in its frame, Mario can jump into the Boo's portrait to warp to the third floor of the mansion. After warping Mario and his partner to the third floor, Boo's Portrait tells Mario to give Lady Bow his regards.
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