I've imported a number of TV shows from my iTunes library. All files contain full metadata including poster images. However the poster images do not appear anywhere in Plex except when you drill into a specific season or episode, at which point Plex displays a poorly cropped horizontal thumbnail of the poster art.
Plex allows you to manually upload a poster image but this is absurdly inefficient, especially given that the image must be uploaded separately for every show and every season.
The whole point of poster images is to make it easier and more visually appealing when browsing different TV shows.