This is "Christina of Denmark, Duchess of Milan" by Hans Holbein the Younger. Painted in 1538, it's a portrait of Christina, the youngest daughter of King Christian of Denmark. At the time, King Henry VIII of England was looking for a fourth wife and Holbein, his official court artist, was sent to Brussels to capture Christina's likeness.
The image is a full-length portrait, which was a format preferred by previous monarchs like Henry VI. This style shows both the sitter's countenance and stature. Christina appears in sombre mourning dress, wearing a black fur-lined satin gown over a black dress, and her face and hands appear pale against the deep black that shrouds the rest of her body.
Holbein made sketches during his sitting with Christina, which lasted from 1 to 4 p.m. on March 12th, 1538. He then transferred these drawings to panel using a technique he developed for other portraits. The image pleased Henry so much that it's reported to have improved his mood, even causing him to make musicians play all day.
The painting is an excellent example of Holbein's ability to create the illusion of real presence. He achieves this through his skill in evoking texture and leaving little space around Christina's figure, drawing her closer to the viewer and heightening the intimacy of the image.
Christina of Denmark, Duchess of Milan hangs in The National Gallery on Trafalgar Square, London — free to enter. Point your phone at any artwork there and audioguide.london plays a free audio guide in six languages — no app download needed.