For example: I have a wedding photo of my paternal grandmother's first cousin. It's easy to figure out who the bride is. With a little thinking one can sort out the groom. Okay, so what about all those other folks?
St. Louis, MO
Uncle Bill during his first year, up front. The happy couple second row right. My grandparents, parents of Uncle Bill, third row from left #5 and #6. My grandfather has his eyes closed.
Fortunately for me, I recognized my very young grandmother and grandfather. I later learned that the baby in front was their new first baby, my Uncle Bill.
Assuming that most of the others had the last name McCully, I rooted around in materials left to me by Uncle Bill's little sister. Praise folks who worked in offices! She not only had a list of attendees but of where in the picture these folks appear. Typed.
Typed information is a gift from heaven. My mother left me a nice album of tintypes with hand-written names. Now was that Foust or Forest?
Sometimes you find a group picture where you know who is there but not who is who. For example, I have a photo of two of that grandfather's maternal uncles standing in front of a pillar in Paris, but I don't know which is who!
The worst is having no idea who that person is in that perfect picture. One can only assume, and therein lies great goof-up potential. And then there's the mystery pic with no connection to anyone you know about. I have one of those, found stuck behind a picture of my father's paternal grandmother. Nobody else in that line knows who the guy is, either.