jQuery(function($){
  $("#primary.nav a img").each(function(){ // preload hover images
    var img = new Image();
    img.src = $(this).attr("src").replace(/\.gif$/, "_o.gif");
  });
  
  $("#primary.nav a:has('img')").mouseover(function(){ // image swap on
    var src = $(this).children("img").attr("src");
    $(this).children("img").attr("src", src.replace(/\.gif$/, "_o.gif"));
  });
  
  $("#primary.nav a:has('img')").mouseout(function(){ // image swap off
    var src = $(this).children("img").attr("src");
    $(this).children("img").attr("src", src.replace(/_o\.gif$/, ".gif"));
  });
});