They've done this before, lots of times now, using stock photos of royalty, from Princess Diana to Princess Anne, and when it comes to William and Catherine, even Graydon-era Vanity Fair had to buy in their cover images.
I don't even think it's a particularly nice image of her, I find the visual message very heavy-handed, choosing a shot with an uncertain facial expression, sombre colours and with her earring askew. It's like when tabloids cherry-pick a random shot to support whatever narrative they're selling that week.