var myImages = new Array(
"mission_on.gif", 
"mission_off.gif", 
"who_on.gif", 
"who_off.gif", 
"how_on.gif", 
"how_off.gif", 
"navbg.gif", 
"tl.gif", 
"tm.gif", 
"tr.gif" 
);

for (var i=0; i<myImages.length; i++)
{
	var imageName = myImages[i].split(".");
	imageName[0] = new Image();
	imageName[0].src = "./images/"+myImages[i]; 
};
<!-- preload images END-->
  
function imgswap(name, type)  
{
	eval("document." + name + ".src = './images/" + name + "_" + type + ".jpg'");
}

var popUpWin=0;
function popUpWindow(URLStr, width, height, added)
{
  if(popUpWin)
  {
    if(!popUpWin.closed) popUpWin.close();
  }
  
// centers the window
    if (window.screen) {
                l = (window.screen.availWidth * 0.5) - (width * 0.5);
        t = (window.screen.availHeight * 0.5) - (height * 0.5);
		}
 if (added!=""){added = ",resizable,scrollbars,"+added;}
else {added = ",resizable";}	
 
  popUpWin = open(URLStr, 'popUpWin', 'toolbar=no,location=no,directories=no,status=no,menub ar=no,scrollbar=no,resizable=no,copyhistory=yes,width='+width+',height='+height+',left='+l+', top='+t+' '+added+'');
}