This is the portrait of Gertrude Vernon, Lady Agnew, painted in 1892 by John Singer Sargent. This currently hangs in the National Gallery of Scotland, Edinburgh, where I saw it in person a few years ago. It is just as beautiful in person as it is in this picture.
Sargent’s portraits capture not only the physical qualities of his sitters, but to a large extent their character. I would guess Lady Agnew was a lively, intelligent, restless kind of person.