[Hampone]

That's what I mean, I can see navigate to the file I want (.jpeg, .png, etc), see the poster and choose it but nothing happens.

You are most likely encountering the 24 hour cache issue.

If you try to replace poster.jpg with another poster.jpg file (or any two images with the same filename), then it takes up to 24 hours for the change to take effect when Kodi re-checks the image hash. 

Ah. I just did a test on one of my sets. I went to my file manager on the Shield (X-Plore), searched for the folder and changed the picture I wanted from "folder.jpeg" to "poster.jpeg". Opened Kodi and re-ran the addon and the movie set now displays the correct poster. Seems I HAVE to rename it to "poster." or it won't work. Thanks Karellen. I appreciate your timely response (thought I would have to wait months for one :-)