Regards the order of posts on a thread: I don't think there's any forum I've ever been on where "newest post first" was the default ordering. Unlike in a Twitter, FaceBook, or Instagram feed, threads on a forum are usually a conversation/discussion that moves from one post to the next: it's intended to move you from oldest to newest so as to articulate the discussion's logical flow.
I can see doing "newest post first" on photo-oriented threads ... that's an environment much more like the aforementioned Twitter, FB, and IG environments. However, I suspect it also means that many will look at the latest two or three photos in a thread and ignore the rest, which is what commonly happens on T/IG/FB.
My feeling is to leave it as it is, just as it was with the old forum software and every other forum I'm on.
G