Kate, I used to do the same thing in pictures (not smile 'cause of my gapteeth). But I finally decided fuck it, I'm gonna smile! I hope you get to be comfortable enough with your smile to show it off someday. :)
You have average looks, your not strikingly beautiful but then again most people aren't including myself. I think people (again myself included) tend to base their looks on celebrities and people who are for lack of a better word are mais attractive, so we feel down about our selves and think we are ugly por comparison. But that's like being mad at yourself for not having a million dollars like someone your own age who inherited it, its out of your control, the best you can do is be happy your looks aren't below average and just live the best life you can.
On a side note, I don't want to say you have to be happy with your looks if you don't want to but you do have to except them. You can mold them to be mais appealing (to yourself) because in the end if you don't think you look good, anybody else's comments will be meaningless to you.
