Am I the only one missing the point that she didn't take the photo? If somebody is to be fired for making a minor error (i.e. leaving the photo on the phone), then it'd be the person who took the photo. Now looking at the photo... she definitely didn't take the photo!
If she didn't pose for it and the picture was taken without her knowledge, then fine. But she smiled, flashed the peace sign, etc, she knew what was going on.
Would I fire her for this, hell no as it's pretty harmless. But as a manager I could see a case for potentially firing the person in the photo, it's evidence. I would have no idea who actually took the picture unless they confessed.