Some say those pictures alone did a great deal to enhance the monarchy's image as the country headed into World War II, helping ensure that Britain didn't follow so many of its neighbors down a republican path.
I'm not sure if Annie Leibovitz's recent portrait of the present Queen quite reaches those heights, but it is awfully atmospheric.
I find its combination of the regalia of royalty - the tiara, the furs, the room (the same White Drawing Room in which her mother was often snapped) - and the reality of a serene old lady (for that is, although it somehow seems surprising, what she is) really rather poignant.
The stillness; the turbid landscape outside (both like that in many classic portraits and somehow metaphoric of the world beyond the Palace gates); the desaturated light; and, just off center, a figure whose inscrutable expression reminds us that she is perhaps both the most famous and, comparatively, most silent person in the world.
It's rare, I think, for a photo portrait to carry as much weight as a painted one used to; this, it seems to me, does.
I think she's thinking of her mother.