This site includes an easy to understand history of 19th century photographic techniques, along with photographic examples.
Photo Tree has an archive of 1,000 photos that you can use to compare your photo and help yourself figure out the date a photograph was taken.
No names were printed on the back, no familiar faces looked back at her so she did what she felt was her only reasonable option, she threw them away.
Now, after I picked myself off the floor, I tried to explain to her that there were ways to identify photos and perhaps in the future opportunities would arise that would help her figure out these long lost family members.
Edin Photo, is a web site that includes a very detailed description of all types of photography processes.