Well, probably the photo is supposed to be taken with the Hangover app in the first place. I don't know app development, but does an app that accesses the camera _have_ to store the photo alongside photos taken with the normal camera app?

My impression was: group of friends are in vegas having a "Hangover (the movie)" style night and agree to only take photos with this app. That way, come the morning they can all review them, but only when they're all together.

Obviously, if they take photos with their normal camera, or if all apps have to save photos to the filesystem and can be seen by the built in photogallery app, this doesn't work. Maybe the app uploads them to the Hangover site and then automatically deletes them from the phone?

It might work if they delete the original photos (if that's still possible on modern devices).