If there is only one subject in a photo it is very difficult to establish their identity, in a group photo, based on comparing the subjects with census returns and then trying to date the clothing etc, it can be alot easier.
It does help though, if by looking at the events of the lives of our ancestors, we can decide on a reason for a photo being taken, A special age, liek a 21st, or a special event, like confirmation would help to give a reason to spend money on what was quite a luxury. If it was a wedding, there would be two people on the pic, a single photo does tend to suggest a birthday, when was she born?