button = new Image();
button.src = "index/button.png";

button2 = new Image();
button2.src = "index/button2.png";

function button(obj)
{
 if (document.getElementById) {
  document.getElementById(obj).style.backgroundImage=button.src;
 }
}

function button2(obj)
{
 if (document.getElementById) {
  document.getElementById(obj).style.backgroundImage=button2.src;
 }
}