I love analyzing old photos. I've given talks on identifying old photos but, Colleen Fitzpatrick over at Forensic Genealogy, http://www.forensicgenealogy.info/ knows how to really identify pictures. Anyone who has seen her work knows that she can squeeze every shred of information from a picture.
Well, she has information on her website so that you too can learn how to determine what time of the day and day of the year a picture was taken (obviously, this is for outdoor pictures). You can learn how to measure shadows and how to use shadows to determine the time.
Want to learn more? I highly recommend her book, Forensic Genealogy, available on her site. She is also speaking at the Palm Springs Genealogy Seminar at the end of this month, see her website for details or contact me and I will email you the flyer.