Caught in Spam Filters and Posts Not Getting Traction

I found this guy on Twitter who makes renders of video game characters. With his permission, I reposted some of his renders to the Rainbow 6 subreddit and they got a lot of upvotes. I told him that he should post his stuff to Reddit for additional exposure, so he made a Reddit account and started posting.

However, his posts seem to be getting caught in spam filters and the ones that don't seem to not get any traction at all. I also have a theory that posts made by people who haven't joined the particular subreddit get heavily suppressed, but I'm not knowledgeable about how Reddit works. Do you guys have any advice for him?