Last year a friend couple uploaded photos of their one-year son (I understand it were those "bathtub photos" parents like taking to embarrass the children when they grow up) just to get their paid account blocked. They panicked because they used Skydrive to "securely" store lot of pictures, including their travels, honeymoon, etc. The husband was a bit tech-savvy, but I failed to convince him to always PGP-encrypt the files and rename them to non-significative names before uploading. At least, I made them never trust these services again.
I, personally, never use these file storage services to hold anything but encrypted data assuming that such data can disappear overnight without any warning.
Hmm what you describe conflicts with what u/m0dest mentioned above. I'd like to hear word from him/her about the matter. If your friend's content wasn't shared, the the image must have been someone processed for known nudity patterns
I suppose Microsoft started using some fuzzy/NN/heuristic/etc. pattern matching algorithm to identify pornography, so it caught the photos my friends uploaded even if they didn't came from a know database.
I just use GnuPG with some specific temporary keys (eg. "Project P File Transfer Key"). All transfer are among desktop/laptop machines, so mobile apps are not a requirement. And I don't use GDrive to sync data to my phone.
I, personally, never use these file storage services to hold anything but encrypted data assuming that such data can disappear overnight without any warning.