Where I found it I don't remember because it was some Google site that I hadn't signed up for. I know, they're all connected, but I didn't even know it existed, much less that personal photos could go there.
My frustration comes from the fact that I had already visited several timesevery configuration and options page of the products that I use, choosing not to share anything anywhere with anyone and, god forbid, never backup phone and still it got there.
I have an idea why it could happen: a combination of "sync" option in the phone being activated by an accidental click with a lot of "creativity" in Google's part.
I'm done with this shit. When the time comes to buy a new phone, I will take some drastic measures. Maybe a dumbphone with tethering paired with a tablet, I'm not sure, but not this again.