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 toi get to be comfortable enough with your smile to montrer it off someday. :)
toi 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 célébrités and people who are for lack of a better word are plus attractive, so we feel down about our selves and think we are ugly par 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 toi can do is be happy your looks aren't below average and just live the best life toi can.
On a side note, I don't want to say toi have to be happy with your looks if toi don't want to but toi do have to except them. toi can mold them to be plus appealing (to yourself) because in the end if toi don't think toi look good, anybody else's commentaires will be meaningless to you.