Girls make ugly/weird/unusual faces in ALL of their photos so when they look at them later, and see their facial flaws, they can blame it on the face they were making. If they had made a regular face, they would have no excuse.
Some people pretty much have nailed it. Some women make that kissy-face to maker their face slimmer, hide imperfections in their features, make it look like they weren't being serious about looking good in the pic, portray that they have some sort of soul behind their superficial lives filled with drinking and sleeping around.
The reason women make that kissy-face in pictures is because they use it as some kind of mask. They want inflate their egos by showing how "hot" they are in pics, but at the same time they don't want to be known as an attention whore, so they do that as some type of psychological cover.