How to add an image below the header on the category pages.

Hi

I am trying to add different images or banners in different category pages.
I’ve read here that 1 easy solution is this:

“put the HTML for the banner in the Category Description field. Then just make sure your theme’s category.php has the code to DISPLAY CAT DESCRIPTION HERE.
to do that, paste this on the category.php file, but i can’t it!”

i am using a theme called “modularity” and my directory looks like this:
public_html/wp-content/themes/modularity
and inside this folder there’s no category.php file there.

thanks.

gaby