Instructions
Play the game until you reach Oswald's Throne Room. Your companion, Gus, will suggest that you restore all the paintings in the area.
Make your way down the long hallway until you find a damaged painting on the wall. Press and hold "B" to shoot a paint stream and restore the painting.
Search the area for other damaged paintings and empty frames. There are a few smaller painting that are hidden in the corners of the room. Shoot paint streams at them until they're fully restored.
Turn down the right corridor at the end of the long hallway. Not long afterward, a giant ball will drop and roll down the corridor toward you. Run back into the throne room to avoid it.
Take the right corridor again. Now that the ball is stopped, you can squeeze past it. At the top of the corridor, you will find a balcony where the rest of the paintings are located. Shoot a paint stream to fill in the remaining paintings.