I've never tried it but am I correct in thinking that if you post a Gallery image on a post, it automatically goes "Under Review" and has to be accepted by the Devs to be uploaded and seen by other users?
No. If PDFs can post their background pics, anyone can.
That's why I was asking because that exact thing is what I was curious about verifying. I've just blocked the 4th one this week because I've opened the app, scrolled like, not even 5 seconds and seen literal uncensored background pics. It's sickening