Having thought through the rules governing the aesthetics of hanging pictures, I have come to the conclusion that most of it boils down to people worrying about things falling on them.

Pictures of equal size generally work best when aligned horizontally side-by-side. Whether they are landscape format [fig. 01] or portrait [fig. 02], they work best when side-by-side.

The exception to this is where the space available dictates that another solution is more logical.

So, hanging same-sized, same-shaped pictures is easy, but what about when we start to mix it up a bit?

Three simple rules apply:

1) Larger/landscape pictures always go above smaller/portrait images.

2) Align Horizontally from the top.

3) then Vertically from the middle unless there is single column, in which case align from a side logically suggested by the space being occupied.

Here's an example of the background dictating usage of space.

Special exceptions