From eBay; spots in background removed with Photoshop and sepia tone removed by gogm.
Mary wears a floral print dress with leg-o-mutton sleeves, the same as in this and this image, while holding her baby, presumably the future Edward VIII.
Keywords: Gunn & Stuart, Queen Mary, Victoria Mary Augusta Louise Olga Pauline Claudine Agnes, Mary of Teck, May of Teck, Princess, Queen, Teck family, Windsor family, British, curly coiffure, pouter pigeon bodice, high enclosing neckline, long tapered leg-o-mutton sleeves, bracelets, shallow vee waistline, close skirt