You can find most of the info on adding custom artwork here.
I think what you want is in the section " Overriding TV series, season, and episode artwork"
Also, you will want to turn off “Embedded Metadata”. That is for metadata that is actually encoded into the file and unless you have encoded the file it will often will be poor quality or more often advertising for the capper.
Edit to add:
If the season doesn’t work like you want try adding the poster for
So is there a similar mechanism for adding a customer poster artwork for a movie? I have a couple of movies in my Infuse library that Infuse is unable to find movie artwork for? Would be great to be able to add a file called ‘movie name-poster.jpg’ into the movie folder and have Infuse pick it up?