if (document.images) {

var button1_up = new Image();
  button1_up.src = "images/rollover_1.jpg";
  
 var button1_over = new Image();
  button1_over.src = "images/rollover_2.jpg";
}

function over_button() {
  if (document.images) {
    document["buttonOne"].src = button1_over.src
  }
}

function up_button() {
  if (document.images) {
    document["buttonOne"].src = button1_up.src
  }
}