I'm trying to complete the quest to gather all 3 paintings. I tried getting the angel painting in the haunted mansion, but there's a problem. I can't pick it up. When I mouse over it, it simply says portrait. I have picked up the other 2 paintings with no problems, but it's just the angel painting that I can't pick up. Is this a bug or is there some "way" to picking it up?

There is a known bug in the game. Once you remove the Portrait, it can be seen on the wall again. The new Portrait cannot be taken from the wall (but the one you took down remains in the inventory.). It is as if you got the second part of the bug without removing the Portrait. Maybe if you talk to a Seer he will give you a copy with the game assuming you lost it. Other than that, only trying the Mansion again comes to mind. If there's a cheat code to get the Painting others would have to help you.