It's tough to figure out because it involves a skill not used often in this game. The level has two parts separated by the fog doors and loading screen. If you're standing in the first part in the room where the painting is, go up the stairs. The one door leads to the fog entrance to the second part, but don't take that. Instead go to the left, past the dude standing there, and follow the halls till you get to a room that seems to be a dead end. I think there's a treasure chest. There is also a crate. Run/walk into the crate till it moves and a crawlspace is revealed in the wall. Through there you'll be able to get Mortimer the Mad's portrait, which is a special loot. Easy to miss, since rarely do you have to push things around, unlike games like Tomb Raider, for instance.