Custom Featured image

Hi,

I wanted to know if it’s possible to add a custom image based on categories for each post.

My problem is that I have mutli authors blog and most of the time they forgot to add featured image which make the blog looks dodgy, so I was wondering if I could add a few lines somwhere that add a feature image to a particular post based on it’s category, so I am avoiding lost of time checking if things are done correctly as this is becoming annoying, yet I don’t want to annoy authors with these little problems.

thanks in advance