function setImgOn(item)	{
 imgObj = eval("document."+item);
 tmpSrc = imgObj.src;
 newSrc = tmpSrc.substring(0,tmpSrc.length-4) + "_on" + tmpSrc.substring(tmpSrc.length-4,tmpSrc.length);
 imgObj.src = newSrc;
}
	
function setImgOff(item){
 imgObj = eval("document."+item);
 tmpSrc = imgObj.src;
 newSrc = tmpSrc.substring(0,tmpSrc.length-7) + tmpSrc.substring(tmpSrc.length-4,tmpSrc.length);
 imgObj.src = newSrc;
}
	
function ImageLoader(args){
 var PATH = "images/";
 imgNum = arguments.length
 if(document.IL = 'undifined')
  document.IL = new Object();
 for(i=0;i<imgNum;i++)  {
  document.IL[i] = new Image();
  document.IL[i].src = PATH+arguments[i]; 
 }
}

ImageLoader("nav_home_on.jpg","nav_about_on.jpg","nav_itinerary_on.jpg","nav_contact_on.jpg");