var objNumber = 0; 

function cacheImgs(name, img1, img2) {  
  if (browserOK) {     
    imgs[objNumber] = new Array(3);
    imgs[objNumber][0] = new Image();
    imgs[objNumber][0].src = img1;
    imgs[objNumber][1] = new Image();
    imgs[objNumber][1].src = img2;
    imgs[objNumber][2] = name;
    objNumber++;
  }
}
function on(name){
  if (browserOK) {
     for (i = 0; i < objNumber; i++) {
      if (document.images[imgs[i][2]] != null)
        if (name != imgs[i][2]) { 
               document.images[imgs[i][2]].src = imgs[i][0].src;
        } else {
               document.images[imgs[i][2]].src = imgs[i][1].src;
        }
    }
  }
}

function off(){
  if (browserOK) {
     for (i = 0; i < objNumber; i++) {
      if (document.images[imgs[i][2]] != null) 
        document.images[imgs[i][2]].src = imgs[i][0].src;
    }
  }
}


cacheImgs("button1" , "I/b1.jpg" , "I/b1a.jpg");
cacheImgs("button2" , "I/b2.jpg" , "I/b2a.jpg");
cacheImgs("button3" , "I/b3.jpg" , "I/b3a.jpg");
cacheImgs("button4" , "I/b4.jpg" , "I/b4a.jpg");
cacheImgs("button5" , "I/b5.jpg" , "I/b5a.jpg");
cacheImgs("button6" , "I/b6.jpg" , "I/b6a.jpg");
cacheImgs("button7" , "I/b7.jpg" , "I/b7a.jpg");