Instructions
Walk into the main hall and find the three portraits of Quidditch players on the three floors. One is located on the top floor, one on the middle floor and one on the bottom floor. Walk to the bottom-floor portrait from whichever side of the screen you entered on -- you can enter on the left or right.
Shoot the painting with your wand using your default attack spell. Watch the painting react to your shot. Wait for it to throw the ball out of the frame of the picture. Walk up the floating staircase to the top floor. Go to the Quidditch player portrait located here.
Shoot the top floor player portrait with the default attack. Wait for it to throw the ball. Walk down the floating stairs to the middle floor and find the Quidditch player portrait located there. Shoot the painting with your default attack. It will throw the ball to the dog painting on the top floor.
Go back up the stairs to the top floor. Find the dog painting located here. It will have opened and deposited the dog's bone -- which will have turned into a gold brick -- on the ground. Walk over to the brick to collect it and finish the puzzle.