Need submenu megamenu

Hi everybody,
I have about 1 month in wordpress and about 5 – 6 years in html. I installed recently a template called plumbelt-lite.1.2.4 from wordpress and i want to make from submenu (12 items), 2,3 or 4 columns. In plus I want to add a picture together with the submenu.

If this is possible, can you please help me?

I will put the css code here in case of some ideeas:

Personally I try different codes and changes but still didn’t work.
Thank you very much for your patience.