A large number of templates contains image menus as icon menu, for example JM-Gardern-Art (submenus).
I'm going to show you how to configure it.
First of all, you have to copy your icon images to: images/stories directory of your joomla installation.
Then navigate to you menu and edit link, see the example screenshot:
Choose a proper image from the selectbox.
Then navigate to Extensions -> Module Manager -> Your Menu Module and edit the module.
1. First parameter from Module Parameter tab to set is:
2. The second ones have to be selected from Other Parameters tab
Now publish your menu module.