Yes, Alex advice was very good and I followed it myself. Before I had all video (.avi) files in one folder and adding cover art was cumbersome. Now, I have for every movie a separate folder, named according to the movie title and inside the movie folder one or more video and image files. I use the option "Add cover art from file".
This has the advantage than I can have several movie files (or episodes) belonging to the same movie or series in one folder.
Of course some more automation would be handy, I added an extra field for actors and I am contemplating moving the director to a separate field director (now it's the artist). In the genre field I make distinctions such as movie, tv-movie, tv-serie, tv-recording and use a field subgenre to do the proper genre distinction such as comedy, drama, fantasy, science-fiction, romance. This allows multiple choices which is not possible with the genre field. I prefer the more squared images if available from the respective audio recordings as they look much nicer than the dvd covers.
In general I would like to have more proper distinction of the different media types in MC (fields, views etc), real separate database schemes for different media types, but in general one always finds some working solution.
Retrieving cover art is quite easy. Enter the movie title in Google-Pictures and select the one that you like best. In most case you will find a proper image. They don't have to be huge as you need them only as templates for thumbnails.