In my opinion, firstly, determine the value of the photos the same criteria as in the painting /graphic:
author name (its significance), artistic and aesthetic value, the year /period of creation, size, state of the snapshot.
In addition to aesthetics can determine its value the moment itself portrayed: an important historical event, person, or His Majesty the moment, seized the photographer.
It is important to time-stamp is made it is valuable, if made within the period of the snapshot.
And your personal attitude to the picture.
(if it is a photograph of your grandparents or a beautiful young mother, she is priceless
)
This is in general.
I think the experts are correct me or add to it.