Wiki says:
"Download covers from TMDB and display them all in My Films."
How does it work? A cover is displayed randomly in films list? Where are them stored?
It's magic!
The covers are stored in the folder you select for Cover images on Artwork tab in Setup. They are named similar to fanart images (e.g. i.am.number.four [-1213360075].jpg). I'll try to make that clearer in Wiki, thanks.
So far they do not display randomly, however in Details Window you have an option on Context Menu to Change Cover. IIRC it also shows you how many covers are currently available for that film. So whenever you get bored with a cover, or don't like one, you can just download them all from TMDB and select one you prefer. Of course, you can always use the Update Feature to download a different cover from another website using a different grabber and then just choose to update the picture property. We do plan to have some grabbers just for nice covers, especially the hard to find ones that often are not on IMDB or other websites.
Why do I feel another feature request coming?
EDIT: Updated Fanart and Covers Wiki page - can you let me know if that is clearer now?