First of all, this picture attracts attention. I was just browsing pictures, and this was one of the first on the results page I wanted to click on; nice job on that.
Time for me to be picky The shadow under her hair looks a bit too sharp. If the light were bright enough, it would be have a more defined corner (like the hair). My eyes got confused when I saw the slightly rounded corner of the shadow with a solid shade. It could have been lighter and more fuzzy, or if you wanted a brighter light, the shadow could've been the exact shape of the hair.
I'm no artist, but the fact that the air-space under her neck is shaded looks funny too. I'm honestly not sure why, it just looks like it it shouldn't be there.
Everything else looks great. I opened an HD an HD video and paused to compare. Those two things are the only things I can notice, and they don't stand out. Great job.
Wow! this caught my attention i just had to write a critique about it!
It is soooo clever how they put sparkles in her, and her last name is sparkle! The effect you put on her looks absolutely amazing! The picture you chose of her is probably the best picture I've seen with her! It also reveals her happy side and not her "not amused" side all the time! You did remarkably well at this and you should be proud by this masterpiece! Twilight is the best pony i know and you made her look astonishing on this picture! ;D