Teri Mattson was on her fourth day of rummaging through old photographs when she came upon it: an amber-hued photo of a motorcycle carrying two riders as they approached a checkpoint in Central America. Their backs are to the camera and a dusty haze surrounds them as they edge closer to the unknown barrier.
Ms. Mattson, a longtime West Marin resident who now spends much of the year traveling, was searching for a photograph to…