if (document.images) {
   img1On = new Image();  img1On.src = "images/but-about-on.gif"; 
   img2On = new Image();  img2On.src = "images/but-ourbus-on.gif";
   img3On = new Image();  img3On.src = "images/but-servprod-on.gif";
   img4On = new Image();  img4On.src = "images/but-career-on.gif"; 
   img5On = new Image();  img5On.src = "images/but-contact-on.gif";
   
   img1Off = new Image();  img1Off.src = "images/but-about.gif"; 
   img2Off = new Image();  img2Off.src = "images/but-ourbus.gif";
   img3Off = new Image();  img3Off.src = "images/but-servprod.gif";
   img4Off = new Image();  img4Off.src = "images/but-career.gif"; 
   img5Off = new Image();  img5Off.src = "images/but-contact.gif";   
}

function imgOn(imgName)
{
   if (document.images) {

      document[imgName].src = eval(imgName + "On.src");       
   }
}

function imgOff(imgName)
{
   if (document.images) {
      document[imgName].src = eval(imgName + "Off.src");
   }
}