I don't consider it the peace sign when it is placed above someones head. I heard it started in Japan and spread to China afterwards. It might mean V for victory which is ironic because I imagine the Japanese started it when they were in China during WW2. It is also another example of the Chinese inability to be original and individual. In the Philippines the girls always turn their heads to one side and pout a little. Photo taking is a strange activity in Asia.

Don't put too much thought into it...

It stands for "yay" or "yeah" here, which is why you'll see people doing it when they get some good news or things... Nothing more....

It means victory

Most people don't know what to do for an informal photograph so I suppose a peace sign is rather neutral and doesn't cause any contempt. What I don't like is when foreign teachers have to act like they are dealing with kindergarten kids when a group photo is taken!! I'm not sure why they do this, but it makes me cringe!

I think it's kinda like the Ricky Bobby "I'm not sure what to do with my hands" gesture

http://www.youtube.com/watch?v=QqhkdHlCHLk

Actually I believe they got it from when Americans used to do it a long time ago. (Now young people in America kind of turn their "peace" signs upside down or sideways , but it started upright). Several people from different Asian countries have said/confirmed this .

it's a friendly gesture, where using the single finger is not,, oddly at home you are more likely to get the single. even from litte kids

no reason, just for fun.that maybe the most easily gesture people could post.

When we stick two fingers up in England, it means we still have them, whereas the French had lost theirs (because we chopped them all off).
This gesture is still widely used by taxi drivers gesticulating to other road users and other generally frustrated UK citizens.
It means Victory, it's nothing to do with peace.