atom()->setContextArgs('primary_menu', array('walker' => new ThumbMenuWalker()));

class ThumbMenuWalker extends AtomWalkerNavMenu{

  // register the thumbnail size, 64 pixel square, cropped
  function __construct(){
    add_image_size('nav-thumb', 64, 64, true);
  }

  // override start_el
  function start_el(&$output, $item, $depth, $args) {

    $classes = empty($item->classes) ? array() : (array)$item->classes;

    $classes = join(' ', apply_filters('nav_menu_css_class', array_filter($classes), $item, $args));
    $classes = $classes ? ' class="'.esc_attr($classes).'"' : '';

    $output .= "\n<li{$classes}>";

    $attributes  = !empty($item->attr_title) ? ' title="'.esc_attr($item->attr_title).'"' : '';
    $attributes .= !empty($item->target) ? ' target="'.esc_attr($item->target).'"' : '';
    $attributes .= !empty($item->xfn) ? ' rel="'.esc_attr($item->xfn).'"' : '';
    $attributes .= !empty($item->url) ? ' href="'.esc_attr($item->url).'"' : '';

    $excerpt = $thumb = '';

    // 2nd level+
    if($depth > 0){
      $post = new AtomObjectPost($item->object_id);

      // thumbnail image
      if(post_type_supports($item->object, 'thumbnail'))
        $thumb = $post->getThumbnail('nav-thumb');

    if($thumb)
      $attributes .= ' class="clear-block"';

      // short description from the content, 100 is the max. character count
      $excerpt = $post->getContent(100, array(
        'cutoff'       => 'sentence',  // cut off just after the sentence ends
        'allowed_tags' => array(),     // no html
        'more'         => false,       // no 'more' text / link
      ));

      if($excerpt)
        $excerpt = "<small>{$excerpt}</small>";
      
      atom()->resetCurrentPost();
    }

    $content = $args->link_before.$thumb.'<span class="tt">'.apply_filters('the_title', $item->title, $item->ID).$excerpt.'</span>'.$args->link_after;
    $output .= apply_filters('walker_nav_menu_start_el', "{$args->before}<a{$attributes}>{$content}</a>{$args->after}", $item, $depth, $args);
  }

}

The CSS:

.nav img, .nav .no-img{
  float:left;
  margin-right: 5px;
} 

.nav .tt{
  overflow:hidden;
  display:block;
  font-weight:bold;  
}  

.nav small{
  font-size:11px;  
  display:block;
  margin-top: 5px;
}

The result should be something like this: