Plex had to remove posters for many movies due to take down requests from the rights owners. For a Few Dollars More may have been one of the affected movies.
If you edit the movie & select Poster and see just a screen capture or no images, then it is one of the affected movies.
Plex is in the process of moving to a new artwork provider: Movie artwork updates
Try refreshing the metadata for the movie or performing the Plex Dance.
If neither works, you can add your own poster to the folder containing the movie. See Local Media Assets - Movies.
Example:
/Movies
/Batman Begins (2005)
Batman Begins (2005).mkv
poster.jpg
