Why do 'party people' all over the globe make this sign when posing for drunk photos?
If I correctly remember, the sign is called
cornuto, and it means among Italians that a person's to which the sign is aimed at wife cheats him. It's also used to banish "the evil eye".
In modern culture it's associated with (heavy) rock music, and it's seems to indicate your general approval of the band playing, and that you're having good time. One reason for using it is probably that you have to hold your hand somehow when you're vawing it in the air, and the other viable solution is fist.
When you think drunken people in photographs, they want to show they're drunk and having good time,* so they have to:
1) twist their face to a cheerful shout or ironic grin
and
2) lift at least other one of their hands.
Again lifting hand poses problem: it has to show some sign. Usual solutions have been plain salutation (which is pretty much the same thing as just lifting hand) or
the victory sign. These how ever are wimpy signs. The drunken wants his drunkeness to be wild and authority defying, a show of his independence (in one word
rocking). Therefore he must use gesture which is almost hostile, and that also explains the grin he might have.
Notice also how a group of drunken people often shows this kind of attitude in their other behaviour too.
*)
They can also be reluctant to being photographed, in which case they turn away from the camera, show the middle finger to it &c.