$(document).ready(function(){
  
  $('#menu a img').each(
    function(){var i=new Image();i.src=this.src.replace('.gif','_hover.gif')}
  ).hover(
    function(){if(this.src.match(/_active\.gif$/)) return;this.src = this.src.replace('.gif', '_hover.gif');} ,
    function(){if(!this.src.match(/_hover\.gif$/)) return;this.src = this.src.replace('_hover.gif', '.gif');}
  );
});