I know how many families have military photographs, and often how you know that Great Granddad had been a soldier in WW1, but that is where it ends for many (or Great Grandma a nurse etc.). Using my own Great Grandfather I can give you some idea of what you can learn from a picture, and a name written on the back.
In this picture we see my Great Grandfather, we can tell from the photo, he is a private in the Coldstream Guards (the Guards regiments soldier’s were not “Guardsmen” until post WW1, a title conferred for services rendered). We know this from his uniform and his cap badge. We know his name is William Paffett , and this means we can search for his records, through sites such as ancestry (it’s always advisable to check the Commonwealth War Graves Commission, to see if the soldier died in the war…). So with a search of his records, we can find the date he entered the war, that he is entitled to a 1914-15 Star, British War Medal and Victory Medal. From the records we can tell what times he was in France, and what battalion he was stationed with and from here through the National Archives we can access his regiment’s wartime diaries and find out he saw action at Bourlon Wood and the Somme (receiving a shrapnel wound to the head, losing an eye and gaining a metal plate.). So there you go, all right it’s not always quite as simple as that (but it may give you an extra avenue of inquiry to go down, and may be the last piece of the puzzle you need), but the following websites:
http://www.ancestry.co.uk or .com (if you want US records)
These places (especially the badge forum) can help with identifying photo’s and insignia to find more information for narrowing down family pictures. If you have any questions about this or similar, please feel free to email me or drop me a comment 🙂