I can totally sympathise with the "hiding the face" thing when people are taking pictures Atticus. I was never a pretty child (or adult) very plain indeed and I always thought (and still do) that my face would spoil everyone else's photograph and have avoided cameras all my life. The thing is I am right. I am the least photogenic person. I feel awkward and self conscious and it shows through on photos. I blame a lack of confidence as a child. I can't ever remember being told I was pretty, lovely, cute etc. because I simply wasn't.
I agree with twiceasnice that there is horrendous pressure for children and young people to look a certain way. Magazines use photoshop to make even the plainest person look "stunning" and young people simply cannot compete with this, leaving them feeling like "the ugly sister" .
I wonder has your DGD always felt like this Atticus or is it something new? Could someone have made a nasty comment to her at school?
I think you need to encourage her confidence by helping her feel good about herself a little bit at a time. Remind her she has a lovely shaped face, nice hair, lovely long legs etc. Also you could look at some magazines or catalogues together (NEXT is a handy catalogue to have) and let her show you what she likes. You could then gently help her see what shapes and colours might suit her best bearing in mind that sometimes the latest fashions might not appeal to us. Is she allowed to experiment with a little make up? Perhaps a little bit mascara and natural lip gloss? Does she have a good complexion? Could a tiny bit of tinted moisturizer help cover poor teenage skin problems?
I do hope that this is a temporary phase she is going through. It is very sad and I wish you and your DGD well.