Paintings and Item Frames can overlap.
What I expected to happen was...:
When trying to place a Painting on an Item Frame, it doesn't get placed or both things drop as item.
What actually happened was...:
The Painting gets placed on the Item Frame without Problems.
Steps to Reproduce:
1. Build an 1x1 Wall
2. Place Item Frame on the Wall
3. Place Painting on the Wall
Note: You need to aim at the Top/Bottom Left/Right Corner to place it.
4. A Painting and an Item Frame are in the same space
Environment
OS: Ubuntu 12.10 32bit
Java: java version "1.7.0_09"
Java(TM) SE Runtime Environment (build 1.7.0_09-b05)
Java HotSpot(TM) Server VM (build 23.5-b02, mixed mode)
The thing is, item frames can stick everywhere, paintings aren't the problem. You can stick item frames behind buttons, levers, torches, everything.