[unreal37]

They went to the same event, took some photos. And in checking their photos, see that the same girl was in their photos too. Then they saw who she was with, and were able to locate a friend of hers. From locating a friend of hers from this second photo, they located her.

[dshanahan]

"didn't have anyone else in it" - seems no one else was in the photo with her.

[Amadou]

Perhaps she was doing a presentation, so was alone at the front of the room. The photographer, being in the audience, would obviously know which presentation it was and from there its a quick look-up on the list of presenters.