Local episode thumbail files?

[beepbot5000]

Have you tried {title}-thumb.{ext}?

But you should be able to do it by clicking the 3 dots on the show, select edit images and 'upload' your image.  I'll use customs for some movies and place it as the 'poster'.

I ended up remembering that you can add images within the UI like that. The image goes away if you move the episode or anything but it's good enough for just a thumbnail.