No hidden meaning there. Like you said, it's just a pose for the camera. Some people like to make crazy faces for a picture or funny faces. The blowing up your cheeks pose is meant to look cute or funny I guess.

I believe I can answer that. Its linked to an overwhelming feeling of inferiority inherent in the nation as a whole. It dates back to when they were considered less of a person and more a statistic by their government (still somewhat the case but less so). They were taught to be quiet and keep your head down and essentially developed the belief they were less important. On a small scale this doesn't tend to be a problem but when its widespread across the nation as in China's case the issues become far more obvious. I digress,  this pose is a type of fight symbol, as if going into battle. Although they clearly aren't going into battle it shows their defiance. It represents V for victory. Essentially when they do this pose in a picture they are claiming victory over anybody else in the image. That's why when you see one doing it the others will join so as not to appear the weakest or least important. Personally I will not accept being in a picture with anyone who tries this as in my opinion it displays a lack of respect.