Can you add your own metadata to Plex?

To edit the details for a particular piece of metadata: Choose the tab of category you want to change on the left, then click the detail field. Type or paste the changes. For items in the Tags or Sharing area, simply start typing the name.

Where does Plex store custom posters?

Custom movie posters must be stored in the same folder as the movie itself. The movie poster ratio is 2:3, so any file you use (the higher resolution, the better) should have that ratio.