window.onload=makeHoverButtons;

function makeHoverButtons() {
  var e, eElements = document.getElementsByTagName('img');
  var len = eElements.length;
  for(var i = 0; i < len; i++) {
    e = eElements[i];
    if (e.className == 'top_button') {
      e.onmouseover = function() { this.style.display='none'; this.nextSibling.style.display='inline'; };
      e.onmouseout = function() { this.style.display='inline'; this.nextSibling.style.display='none'; };
    }
    if (e.className == 'top_button_hover') {
      e.onmouseover = function() { this.style.display='inline'; this.previousSibling.style.display='none'; };
      e.onmouseout = function() { this.style.display='none'; this.previousSibling.style.display='inline'; };
    }
  }
}