Adding a z-index on rollover with Java script

1

I am using a simple java script for rollovers but I need to add z-idex when I rollover. How would I code this?


var ns4 = (document.layers);
var ie = (document.all);
var ns6 = (!document.all && document.getElementById);


if (document.images) {
  
  // menu
  image1on = new Image();
  image1on.src = "/images/menu/learn_more_on.jpg";    
  
  image1off = new Image();
  image1off.src = "/images/menu/learn_more_off.jpg";

}

function turnOn(imageName) {
  if (document.images) {
    document[imageName].src = eval(imageName + "on.src"); 
   //set z-index here?
  }  
}

function turnOff(imageName) {
  if (document.images) {
    document[imageName].src = eval(imageName + "off.src");
  }
}