What does this grinning gesture signify?
My understanding of it is basically that they are making a face, except it's different to how we usually know it. These are how they do it nowadays:




They are all making a face and saying "bi~". It is exactly the same as how we make a face, but Japanese style. I don't have research evidence to support this, but I have consulted quite a few Japanese people who lived in Japan for quite a while and they all agreed to my statement, so I do think it's very reliable.