Celestia's starting to look pretty good.
If I may though...
Well, it's just one man's opinion, but I cannot see the sun Princes as light-skinned. Luna? You'd better bet she is pale, but not Celestia. Consider giving her brown skin...
As for her cutie mark. Perhaps if you put a layer of greenstuff over her right spaulder to smooth it off that'd be a good place to paint her cutie mark on?
Finally - her eyes are... er, a little bit crazy-looking. I'm damned if I know how to fix that, but it is worth mentioning. (The model itself has closed eyes).
Oh, finally, finally, if you're wondering which model to use as Luna, how about this one?
She's got that suitably dark 'n' creepy vibe to her and the empty left hand is an ideal place to add a gun or some such.
I got mine from here