The peace thing is just copied over from the Japanese. People like to do funny, humorous and fun things in their pictures. Not everyone likes just being sat in front of a back drop and having their picture taken, it feels unnatural and you can't feel their personality through the picture, same.
Of course, some people like that, but it's been proven that if you look fun/look like you're having fun in your dating profile pictures, you are more likely to get someone message you and be more likely at finding a partner.
I can't really say the same for myself, I find every time I try a different pose, it doesn't work no matter what I do because of my horribly freckle covered skin, and I have a big scar on my nostril, so I can only really take pictures on one side of my face xD I never take pictures of myself for this reason