Painting shot is not a Gainsborough...
...as is often cited. it is a portrait by one of his contemporaries, George Romney. The model is Frances Puleston, (or Mrs. Bryan Cooke.)
Very interesting article by one viewer who discovered this, HERE:
http://fun-with-kubrick.tumblr.com/post/101080525324/identifying-the-portrait-in-lolita-and-examining
.