if (document.images) {

            img1on = new Image();      
            img1on.src = "img_1b.jpg"; 
            img2on = new Image();      
            img2on.src = "img_2b.jpg"; 
            img3on = new Image();      
            img3on.src = "img_3b.jpg"; 
            img4on = new Image();      
            img4on.src = "img_4b.jpg"; 
            img5on = new Image();      
            img5on.src = "img_5b.jpg"; 
            img6on = new Image();      
            img6on.src = "img_6b.jpg"; 
            img7on = new Image();      
            img7on.src = "img_7b.jpg"; 

            img1off = new Image(); 
            img1off.src = "img_1.jpg"; 
            img2off = new Image(); 
            img2off.src = "img_2.jpg"; 
            img3off = new Image(); 
            img3off.src = "img_3.jpg"; 
            img4off = new Image(); 
            img4off.src = "img_4.jpg"; 
            img5off = new Image(); 
            img5off.src = "img_5.jpg"; 
            img6off = new Image(); 
            img6off.src = "img_6.jpg"; 
            img7off = new Image(); 
            img7off.src = "img_7.jpg"; 
      }

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");
        }
}