You've already seen the young man above in a post from a month ago when he was hunting along the drainage canal next to my hotel with his friends (if you didn't, it's here). I didn't include this image then because it didn't seem to fit, but it's a good example of a pose many younger Thai strike when having a picture taken. I've seen it far more often being used by males than females, but my guess (based only on my own observations) is that Thai girls are more reserved and "proper" overall, by virtue of their culture. Boys tend to be more rambunctious and prone to bravado worldwide, I think.
I've heard several explanations for this, but I lean towards what I've been told by Thai friends: it's a way of saying law (handsome) or suay (beautiful) if used by a female. I suppose it could also be explained as a way of saying "I'm so cute" or "Aren't I picture-perfect?" when they quarter-frame their face with thumb and forefinger. It's invariably accompanied by a wide grin, and I find it delightful.