How to make thumbnail menu?

Hello,

I want to make WP menu, but instead of text menu it should be thumbnail menu. I have got several static pages and these pages has got thumbnails images and I want to have these thumbnail in menu instead of text.

I was searching some plugin but I didn’t find any. Another option how to do that should be some editig of wp_nav_menu function. But I don’t know, where can I find structure of this menu to edit it and in addition I don’t know event any function which should display thumbnail instead of text.

Have you got someone any idea?

Thanks.