They're celebrities. It's not an easy thing to do to the point where you have such a slim chance of getting hacked as a non-celebrity, you might as well not even think of it.

Honestly... you're VERY safe with google photos. Not only because the photos are private, but also because there are hundreds of millions of people that have nudes in google photos. Unless you or your girlfriend are some kind of celebrity or public figure, or you have a lot of people that hate you, you are completely safe. If you set up two-factor-authentication, you're even safer. Truly, I'm sorry to tell you, all of this was for nothing. (This is coming from a tech guy that knows what it takes to get private information from someone btw)