WordPress: How to retrieve custom fields to use in Menu

I have a challenging question for you guys, I want to create a navigation menu which shows up an image related to the page hovered upon. Now the image would be stored in a custom field saved from that particular page. So what I need to know is how to have the menu (generated using wp_list_pages) retrieve that custom field from each page in the menu.

Also if there is any better approach to create such a menu I would also be interested in knowing it.

Thanks!