Some additional follow-up:
You can also use WebTools-NG to export existing posters and art. It can run on a different host than your Plex Media Server. Click on Assets to see available downloads.
I ran a quick test with WebTools installed on my Windows 10 desktop. I was able to export artwork/posters to my PC from my Plex Media Server running on Ubuntu.
See the link below. You can setup a custom export. Drag the desired fields from Available to Custom.
The Wiki link on the GitHub page has additional information on installation, configuration, etc.