I think there are probably many reasons people don't post a picture or comment on them. I didn't post a picture for a very long time. Just because I chose not to. Some may be afraid "the wrong" person might get wind of them being on a BF site. I know someone who worked for a state agency a few yrs ago, she was on a dating site under woman seeking woman. Her boss found out about it and made her take her picture down, siting that a parent of the children she over saw might be upset if they saw that.
Although I love to look at the gallery I don't always post a comment. Perhaps I have nothing to say, perhaps you are in a relationship with someone I don't know and what is on my mind might be disrespectful, maybe I don't want to bring your attention to me. Fat, skinny or inbetween, everyone on earth complains about their weight whatever it is, everyone doesn't like something about their looks that might make them chose not to post a pic. I think it is just a personal choice and maybe a choice to post a pic might come as one gains confidence in themself and in the site community.
I look at pic posting like I do one of the IM's I use. It makes you put in your name when you sign up and when you IM it shows your name. My gosh I might not want everyone to know my real name considering some of the weirdo's around ya know? Same with pics, I might not want everyone to know what I look like. But to each his own. LOL here I am spouting off and I'm not sure if I have any pics in the gallery right now or not. Lemme go look