[jordan_clark]

You bring up a valid point. Presenting a picture as the only means for authentication removes the benefit of digitized confirmation. Pics on the app store link appear to show a clock of some sort over the picture, but even that could be easily recreated.

[_garrett]

You could add a visual icon as a sort of two-factor authentication. Set the icon to change every few minutes and do a push notification with the current icon to the people at the door.