The artist is not identified where I can find this image, but the posing makes it look like a work by Gerrit van Honthorst.
Upgrade image posted 19 January 2014 also from lisby1's photostream on flickr.
Princess Mary wears a hurluberlu coiffure and an opulent, but plain, dress and limited jewelry in keeping with Dutch tastes of the day. She looks like she is in her teens, dating this to 1647 - 1650.
Keywords: Princess Royal Mary of Great Britain, Mary Stuart, Maria Henriëtte Stuart, Prinses van Oranje-Nassau, Stuart family, Oranje family, British, Princess Royal, Princess, Dutch, proto-hurluberlu coiffure, hair jewelry, necklace, earrings, straight neckline, bertha, brooch, elbow length puffed sleeves, basque waistline, close skirt