By the default the cover thumbnailer is disabled. The reason is that it sends information (movie title extracted from the filename) to online services to retrieve a poster. This information is normally harmless, but it is private, so therefore it's not enabled by default.

In other to use the cover thumbnailer you need to create a config file to white-list locations. If the thumbnail file is a child of one of the locations, it will try to download a poster.

~/.config/tumbler/cover.rc

[General]
# a ; separated list of white-listed locations
Locations=/home/yourname/movies/;/home/yourname/downloads;sftp://yourhost.com/home/bla;
 
[TheMovieDB]
# API key to use themoviedb.org information
#API-key=your-private-api-key

The cover thumbnailer supports two different API backends:

• http://omdbapi.com
  The Open Movie Database API by Brian Fritz. The used posters are most of the time retreived from IMDB.
• http://themoviedb.org
  A free and community maintained movie database. This API however required an API key.

After creating the rc file, you can run killall tumblerd in a terminal and next time you enter a white-listed movie folder, you hopefully see poster thumbnails.