This past week my WH's Android phone started posting pictures and videos from his phone to Facebook... all by itself.
I was logged on to facebook watching the videos/pics post on my laptop while sitting in bed next to him, with his phone on my nightstand.
Turns out his Android had a virus. We think it was attached to a work email he had opened.
I tell you this story because I think you need to do a quick virus scan on his phone and tablet. Also, if you know the date the pictures were added, I would go back through his email, including trash folders, and see what was opened.
Another thing to check would be his youtube history...
Sorry you are in this position doesitgetbetter. But I think you need to go full assault on this and research the bejeebies out of it NOW or else that seed of doubt in your mind is going to grow. Best to leave no stone unturned now rather than wait and risk not being able to do the research.