Instructions
Head down the stairs and look on the library floor after Mickey speaks to Madame Loena. You will see the first skull. Press the "A" button to paint the skull, then head back upstairs.
Approach the sliding bookcases. Use the "Z" button to hit the framed painting between the bookcases with a stream of thinner. It will disappear, revealing the second skull. Press the "A" button and paint in the skull.
Head to the far left of the library and maneuver around the dancing bookcases. Use thinner to erase the wall here and reveal the third skull. Paint in the third skull to complete the task.