Whether it’s artist-poster.jpg, folder.jpg, or folder.png, Plex is ignoring local artist poster images. The library is configured to prefer local metadata, and to use local files only for album art, and that works perfectly. But there is no setting for artist posters, so I’m assuming Plex is supposed to follow the traditional priority of local sidecars, embedded, images, online sources. It doesn’t appear to be doing so.
Here are a few examples. The images on the left are what Plex finds. On the right are the local images:
This is still a problem, or a problem again, if it was ever fixed. And not just with audiobooks. I just ran some experiments with a couple of music artists, and local artist posters aren’t even added to the Artist Edit/Poster tab.
Looking through logs after a ‘Refresh metadata,’ and again after a Plex Dance, I don’t even see mention of the local artist-poster.jpg file (or folder.jpg), so I don’t know if it’s even trying.