Banner or feature image on category archive page

Hi, I’d like to add a featured image across the top of the category pages on my blog – I can’t figure out a way to do this. I’ve tried adding an image to the description field in the ‘edit category’ area but it doesn’t display right – I want a banner image across the top of the page.
Any ideas how I can solve this?

The page I need help with: [log in to see the link]