if (document.images) {            
            img1on = new Image();   img1on.src = "images/bottom_b1a.gif"; 
            img2on = new Image();   img2on.src = "images/bottom_b2a.gif";
            img3on = new Image();   img3on.src = "images/bottom_b3a.gif"; 
            img4on = new Image();   img4on.src = "images/bottom_b4a.gif";			
            

            img1off = new Image();  img1off.src = "images/bottom_b1.gif";
            img2off = new Image();  img2off.src = "images/bottom_b2.gif";
            img3off = new Image();  img3off.src = "images/bottom_b3.gif";
            img4off = new Image();  img4off.src = "images/bottom_b4.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");        
            }
    }