This is applicable for left menus, rather than top menus.
Go to Design Settings, then into the "menu design" area, there is an options called "Menu Footer"
From here you can add content to the bottom of the navigation, like images, adverts, links, text etc.
If you want to put images there, we suggest they not be any wider than the navigation. Remember to resize the images to about 120 pixels wide.
How do I change the name of a menu button? or specify the order of a menu button.
Also options for SEO
...to custom-designed website template and standard websites templates.
Place cursor over "Design Settings" then "Menu design" then select "Menu Design" This will give you the menu management screen
There are a couple of menu style options specifically for showing all menu dropdowns in a big Mega menu box.
You can create custom menu buttons and upload them individually for each page by clicking on rename [Aa] for each page, then browsing for your menu button, then click save
How to add photos or links or other content beneath the menu buttons.
How to change your menu template to allow for CSS manipulation