The 3x4 and 4x4 paintings act strangely. They can be placed even if there is no wall to hang on to below or to the left, but they need empty wall room above and to the right. If one of the top/right blocks is broken, the painting will fall. Basically, the blocks the painting is attached to aren't the same as the ones the painting is covering.
Same thing for the 2x4 painting, except the blocks above and below aren't affected.