Why li:eq() doesn't work ?

Why li:eq() doesn't work ?

Hi I'm trying to use addClass in a img inside the li, and I try to get the depth with a variable for be use in :eq() of the next way, but doesn't work, but if change my depth variable for a simple data inside the :eq, works it, but I can't do it this way because, all images change the class, I need just change the image inside the li.
 
Thanx for your help !
 
 
this is the html
<ul id="MenuItemsZavaSocial">
      <li class="offPicture"><img class="offPos" src="images/diabeticos.jpg" /></li>
         <li class="offPicture"><img class="offPos" src="images/mujer.jpg" /></li>
            <li class="offPicture"><img class="offPos" src="images/medioAmbiente.jpg" /></li>
     </ul>



 
this is the event
 
$('#MenuItemsZavaSocial li').hover(
  function(){
   var depth = this.index();
   $(this).addClass('onPicture');
   $('#MenuItemsZavaSocial li:eq(depth) img').addClass('onPos');
  },
  function(){
   $(this).removeClass('onPicture');






      $('#MenuItemsZavaSocial li:eq(depth) img').removeClass('onPos');

  }
 );


 
but works it if I put something like this
 
$('#MenuItemsZavaSocial li:eq(0) img').addClass('onPos');
 
Why !!!!!!!!!!!
I'm going to crazyyy !!!! :(