if (document.images) {

 function turnOn(imageName) {
   if (document.images) {
     document[imageName].src = "img/b_"+imageName+"_2.gif";
   }
 }

 function turnOff(imageName) {
   if (document.images) {
     document[imageName].src = "img/b_"+imageName+".gif";
   }
 }


 }