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