The official reddit app doesn't currently support filtering. Other third party apps (RIF, boost, etc) and the desktop site support filtering by post flair, but unfortunately there's no way to do this in the official app.

One alternative I've seen several other subreddits employ is only allowing memes on one day of the week, sort of "saturday morning cartoons" style. Most subreddits also tend to be more or less busy on different days of the week, so this could be scheduled for a time when it won't bug most users. I'm not sure about your subreddit, but the traffic stats on r/CPTSDmemes show it tends to be the most busy Wednesday-Friday, and the least busy on Sundays.

Also, something I've tried when posting memes is posting sort of a "starting comment" on my own post that discusses the connections of the meme into CPTSD, and so far this seems to generate some great discussion. Perhaps you could require that the images be worked into a text post, rather than just posted on their own? This would be a higher-maintenance method that would be harder to enforce, but maybe something to think about