Celestia's expresion and Twilight's confused look make me think she just tricked Twilight into becoming her bride for eternity or something. "You didn't think becoming an alicorn had no consequences, did you?" Sorry Twi, looks like Celestia is here to show off her new prize before you get to learn what being an alicorn is REALLY all about.
But hey, that's just my sick mind interpreting things. Lovely piece. Especially like all the shading and detail on the clothing and such. Just great!
Princess Celestia kinda has that look of a powerful, scheming ruler, where she knows everything, but lets some bad things happen so that she can show who really runs the joint. (think lady seto from tenchi muyo.)