How to add poster .jpg manualy (and in sub-folders)

Hi
I´m using Artwork downloader but some of my Swedish movie posters can´t be found by the add-on.
So how can I add a poster manually to my Swedish movies? Where should I put the jpg and how do I name the file?

Another question:
Lets say I have a folder called “Fredrik Kraft” with several sub folders with movies with this guy Fredrik Kraft.
Could I have a poster for the highest level folder where there is no movie file included (just sub folders with all Fredrik Kraft movies in)?

Thanks guys for your support