Sadly, I can't use it in combination with posts import + "auto post thumbnail" plugin which looks for the first image in the post and set it as thumbnail (would be nice to have this included in your plugin.)
I guess that the main reason is that when I import the post, the FPW assigns the thumbnails before auto post thumbnail, therefore I allays get category thumbnails from "FPW" and not the post thumbnails (it skips the post thumbnail if it already has the set thumbnail).
Anyway to run your plugin with "cron" at specific time? So I could run it 1 hour after import?
My suggestion would be to go for
1. "image from custom field" (Any way to implement "custom field" for thumbnails?)
2. check for the first images in the post
3. and then if there is no images to go for category image