There are addons for poster/fanart that you can install to automatically fetch the necessary files.

However, I prefer to choose my movie posters.

Copy the poster jpg file to the folder of the movie and rename it poster.jpg. The folder will appear in Kodi as a poster. Make another copy of poster.jpg and rename it name_of_movie_file.tbn and the movie file will appear as a thumbnail.

A little bit of extra work, but I organize my movie collection in exactly the way I want.