From Wikimedia. The image was extensively spotted, including spots on the lips and her right eye, the tree to the right side of the image, and her dress. I fixed these spots with Photoshop.
Therese von Thun wears a stylish dress from around 1830 with a bateau neckline and big puffed, probably Gigot, sleeves and fan fronted bodice. She wears a lace hat and appears to be wearing a fichu tucked into her bodice.
Keywords: Peter, Therese of Thun, Therese von Thun, Countess, German, curly coiffure, lace hat, fichu, bateau neckline, fan front bodice, Gigot sleeves, cuffs, waist band, full skirt, draped necklace, brooch, wrap