Adding custom header to catergories in yoko theme

Hi, I’m new to wordpress and I was wondering if anyone could tell me how to customise a category with a specific header image using the yoko theme.

I understand that it is not possible to do this on the dashboard and it requires an update to the .php files somewhere. I’ve tried a few fixes from other discussions for other templates with terrible results, heh!

I’m very new to writing code and really just need an idiots version. What code do i use, where do I put it and where do i ammend the code to put the details of my image in?

Any help would be greatly appreciated 🙂