I don't think every picture you take you should try to look "pretty"
your inner beauty shows through regardless of the face you make!
One picture does not define your overall beauty. I know many girls
who won't make a silly face in a picture because they are scared they
will look "ugly".
Have fun, be silly, and don't worry so much about what others think.
As long as it's not harming you, or anyone else... Enjoy yourself :]