How do I swap images on the sub-categories of a pop-up menu?

How can I add a swap image behavior to the sub-categories of a pop-up menu? I know how to swap images for the main categories, but can't figure out how to do it for the sub-categories.

I'm designing a site for an attorney.  he's requested that the navigation include the topic "Areas of Practice" with a pop-up menu showing 5 options: "Family Law, Real Estate, etc."  He wants the image on the homepage to change as the user mouses over each of the 5 areas of practice.  I've often used Fireworks MX and Dreamweaver MX to create pop-up menus with a swapped image for the main topic (in this case, Areas of Practice) ...but I can't figure out how to have the swap image effect work with the seb-menu items (Family Law, Real Estate, etc.).  Help!