In TV mode click "Editor":
There are 2 options:
(1) Create Episode ScreenShot
(2) Rescrape Episode Thumb
First off a minor issue but this sort of thing was and sadly still remains confusing for me, the use of a number of different words to mean exactly the same thing, can I suggest these options be:
(1) Create Episode Thumb from ScreenShot
(2) Rescrape Episode Thumb
You need to decide on a few terms, hopefully use them consistently and hopefully put them in a FAQ...
OK now the main issue is that the "overwrite episode thumb" only works for (2), this is confusing and actually I'm not sure why you'd want this, if you weren't happy you'd cancel...