Custom Walker for wp_list_categories

I need some help with making of one Walker Class. Now I’m having a menu with categories and in the dropdown there are some subcategories. The menu is something like that:

<div class="navigation">
  <div id="nav">
    <?php
    wp_nav_menu( array(
    'theme_location' => 'header-cats',
    'menu_class' => 'nav',
    'menu_id' => 'cat-nav',
    'fallback_cb' => '' ) );
    ?>

    <ul id="nav1">
     <?php wp_list_categories('title_li=&orderby=id&exclude=1,2,3'); ?>
    </ul>
  </div>
</div>

http://imageshack.us/photo/my-images/228/21946748.gif/

I want to show the title of the last article with a small pic of it in that dropdown menu. Now I’m having only subcategories there. I want to be the subcategories AND the last title with + a thumbnail in every category. The menu should be like that:

http://imageshack.us/photo/my-images/339/81352218.gif/

For example there is something like my idea in www.cosmopolitan.co.uk There are showing the latest 2 articles from every category.

Can you help me with that? I’ll be so grateful! Thank you!