The photo is probably by Hansen.
From tumblr.com/tagged/maria-feodorovna; enlarged by half and increased contrast.
Dagmar poses by herself wearing the dress seen in this image.
Keywords: 1866, Maria Feodorovna (1847-1928), Prinsesse Dagmar, Princess Dagmar, Dagmar (kejserinde af Rusland), Marie Sophie Frederikke Dagmar, Мария Фёдоровна (жена Александра III), Glucksberg family, Romanov family, Princess, Empress, Russian, straight coiffure, high neckline, bow tie, collar, jacket bodice, buttons, long close sleeves, contrasting cuffs, over-skirt, flounced crinoline