Background image for submenu

Hello!
I’d like to have an image for the submenu under “Önkormányzat” top level item. Right now i only have a small crest in the bottom right corner, cause the submenu items and that tiny space under the horizontal rule obscuring the image.

I’m using Ubermenu by SevenSpark, and their tutorials didnt really help much.
Any idea for a solution?

Thanks in advance for any reply!

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