Frank P. Walentynowicz

Setting featured images for posts / pages could be very time 
consuming, especially when your media library holds hundreds of 
pictures. Very often we select the same thumbnail for posts in 
particular category. This plugin automates the process by inserting a 
thumbnail based on category / thumbnail mapping while post / page is 
being created or updated. Now bundled with FPW Post Thumbnails.

FPW Category Thumbnails

Assigns a thumbnail based on categoryid/thumbnail mapping to a post / page when the post is created or updated. Built-in FPW Post Thumbnails.

Johan Jonk Stenström

Lists categories, author pages and archives with thumbnails. Use shortcode [categorythumbnaillist 1] where 1 is the category id.
Don’t forget to add Featured images to your posts.

Category Thumbnail List

Lists categories, author pages and archives with thumbnails. Use shortcode [categorythumbnaillist 1] where 1 is the category id.

FPW Category Thumbnails vs Category Thumbnail List