I mean, I’m not saying that we can enforce this as law or anything. I also might be wrong about this. But:
Just as a general rule, I feel like we should not look at pictures of the breasts or genitalia of people who would rather we not look at pictures of their breasts or genitalia.
As a corollary to that general rule, I would add that I don’t see anything wrong with looking at pictures of breasts or genitalia of people who have invited us to do so. There seem to be plenty such pictures for us to get a reasonably good grasp of, like, the diversity of unclothed human anatomy without having to look at people who wish we wouldn’t.
This seems pretty straightforward to me. Yes, the photographer(s) who photographed Kate Middleton’s grainy distant breasts were violating her privacy. But so do people who choose to look at those pictures.
So maybe we can just agree not to? And this goes not only for princesses, I would argue, but also for people who send things to their romantic partners, who turn out to be jerks and release those photos publicly. Or people whose phones are hacked. etc.
In this world where most every curiosity can be satiated, it seems to me genuinely heroic to resist the urge to look at everything that can be seen, and instead to respect the wishes of those who feel violated or hurt by the availability of images they wish were private.