window.addEvent("domready", function()
{
    $('portal').getElements('img').each(function(child) {
        var siblings = $('portal').getElements('img').erase(child)
      
        child.addEvents({
            mouseenter: function() { 
              siblings.tween('opacity',0.5); 
            },
            mouseleave: function() { 
              siblings.tween('opacity',1); 
            }
        });
    });
   
});