function preload() {
 var img = new Array;
 for (var i=0;i<=7;i++) {
  var index = img.length
  img[index] = new Image;
  img[index].src = "images/n_button"+i+".jpg";
  index++;
  img[index] = new Image;
  img[index].src = "images/f_button"+i+".jpg";
 } 
}

function on(img) {
 if (document.layers) {
  var div = img.replace("b","but");
  var target = document.layers.thepage.document.layers[div].document.images[img];
 }
 else {
  var target = document.images[img];
 }
 roll(target);
 target.disabled = true;
}