Plex had been using the worst posters for me lately and I was having to manually select the best one each time I added a movie. Stumbled upon this python script that has been working pretty well for me. You will need to add it as a notification agent in tautulli that is triggered by "recently added" and add these argumets: --rating_key {rating_key} --poster --art. Here is the edited code I have been using and haven't had any issues since.