From a practical point of view the photographs should be centered at eye-level of the intended visitor.

But... there are other aspects:

you can arrange them to form relations between several of them, or to relate them to the room.
http://www.lenarot.de/images/2010/02.jpg

you can place them a way that the visitor is forced to take a certain position
http://4.bp.blogspot.com/_KNvwAfuC47...a+de+Bank1.jpg