I’m quite happy that this feature is finally being added. For the collections that I currently have, I have collection posters for them that I’d like to use instead of the auto generated one. I’m not sure how to change this default, though. On Plex Server, the movies aren’t shown in aggregated collection folders so can’t edit the poster image there.
You change them the same way you change any movie poster. click on the edit button ( the pencil icon) Info how to customize can be found here https://support.plex.tv/articles/201273953-collections/
Sorry for the delayed response as I’ve been on vacation and just got back.
I must be missing something or a step is missing in that support article. I haven’t really used collections at all in Plex due to the prior limitation but I do have my movies with the tag as I was using Kodi / SPMC with my library as well. I can confirm a movie is in a collection by selecting a movie or by seeing the aggregated view when using the client.
What I don’t see is how to edit the image. I’m in the web server interface and I don’t see a way to do this…
“If you do not have collections displayed inline then switch to view by collections”
All I see are options to filter to a specific collection but then that just shows the movie in a collection.
Also, is there a way to have Plex automatically pick up the image?
got it…
“The new collections feature is only available starting with PlexWeb 3.32.2, which at the time of this writing is only available via https://app.plex.tv/”
That was from the Dec 2017 post 
