Photography was in its infancy when the civil war broke out. Many soldiers carried photographs of loved ones into battle. Unlike modern soldiers, few Civil War soldiers had the modern day version of dog tags, and few carried identification. So, if a soldier was killed in battle, and not identified, any pictures he may have had went unidentified also. Such is the case of a picture of a soldier given to a woman to safeguard for him the night before the battle of Shiloh. He never returned for it. Or a small locket found by a soldier in Hampton's cavalry brigade on the battlefield in 1863. The small locket shows a picture of a small boy. It was never identified.
Many such items of unidentified pictures have been turned over to museums in the hopes that they would at some point be identified. When that has not happened, in many cases museum officials release the images to the public in hopes of identification. Most of them have little back story, they were often found by another soldier and saved in hopes of finding the loved ones pictured, or passed onto a stranger before entering battle.
There is the case of two photographs, an enduring mystery. Their images were found among the crumpled bodies on Civil War battlefields, each of a little girl, each posed primly on chairs. One has ringlets cascading down her back, and rouged cheeks. The other dressed in a frilly hooped dress. No one knows the identities of the girls, no one knows the stories they might tell. There was also the photograph of one little girl that was found on the battlefield, between the bodies of two soldiers, one Union and the other Confederate at Port Royal, Virginia. Another was retrieved from a slain soldier's haversack in 1865 on a Virginia farm field days before the surrender at Appomattox.
Typically the photographs have been passed down through generations, ultimately ending up in an attic, or truck, that when emptied out was given to a museum.
Sometimes the story behind an unidentified photo is eventually told, as in the case of a Union soldier who died at Gettysburg clutching a photograph of his family. Widespread efforts ultimately proved successful and his family was traced to upstate New York.
Unlike today, the Civil War did not have the kind of mortuary units that now strive to collect all the possessions of the war dead to return to their families, and with the horrendous loss of life, this would have been impossible.
More than anything these photographs show there was more to the war than combat and death. You have these soldiers out there in all sorts of bloodshed, and he's carrying a picture of his little girl.