I just tried with a single photo of my husband when he was about 10, his mother (then in her mid forties) and his maternal grandparents (then in their early seventies). It was a scan of a postcard sized black and white photo taken in about 1960 of the four of them standing in a line at the seaside. It was not a profession photo, but taken by my OH's father with a Box Brownie, so not of best quality and clarity. As it was full length photo, the faces were very small
.
The software first managed to pick out the four faces from the composite photo, my OH had a 61% match to his mother (he seemed to look more like his mother when younger, but getting more like his father now) and a 36% match to both his grandparents. I'm surprised there was no difference there, I thought he looked a bit more like his grandfather than grandmother.