function ImageChanger() {
  if (!document.getElementById) return
  
  var imageArray = document.getElementsByTagName('img');

  for (var i = 0; i < imageArray.length; i++) {    
    if (imageArray[i].className == 'imgover') {
      var src = imageArray[i].getAttribute('src');
      var suffix = src.substring(src.lastIndexOf('.'), src.length);
      var osrc = src.replace(suffix, '_o'+suffix);
      
      imageArray[i].setAttribute('nsrc', src);
      imageArray[i].setAttribute('osrc', osrc);
      
      preloadImage = new Image();
      preloadImage.src = osrc;
      
      imageArray[i].onmouseover = function() {
        this.setAttribute('src', this.getAttribute('osrc'));
      }
      
      imageArray[i].onmouseout = function() {
        this.setAttribute('src', this.getAttribute('nsrc'));
      }
    }
  }
}
try{
  window.addEventListener("load", ImageChanger,false);
    }catch(e){
  window.attachEvent("onload", ImageChanger);
}