I'm guessing it has to be something to do with facial recognition, since he asked for the picture without knowing its contents (other than that it has her face in it).
From there, if he had an idea of where they went that night, a club or bar, perhaps he could do some Facebook/Twitter searching magic to search for pictures of that night, taken by other people. Then they could look for her face in the background.
Not sure where to go from there... perhaps they got lucky with one of the pictures being tagged with name. Or perhaps they identified everyone she was shown to be associating with, and found intersections in their Facebook friends list? Does it have to do with Facebook?
From there, if he had an idea of where they went that night, a club or bar, perhaps he could do some Facebook/Twitter searching magic to search for pictures of that night, taken by other people. Then they could look for her face in the background.
Not sure where to go from there... perhaps they got lucky with one of the pictures being tagged with name. Or perhaps they identified everyone she was shown to be associating with, and found intersections in their Facebook friends list? Does it have to do with Facebook?