if (document.images) {
  btn_about_us = new Image();
  btn_about_us.src = theme_root + "images/btn_about_us.jpg";
  btn_about_us_on = new Image();
  btn_about_us_on.src = theme_root + "images/btn_about_us_on.jpg";
  
  btn_products = new Image();
  btn_products.src = theme_root + "images/btn_products.jpg";
  btn_products_on = new Image();
  btn_products_on.src = theme_root + "images/btn_products_on.jpg";
  
  btn_resources = new Image();
  btn_resources.src = theme_root + "images/btn_resources.jpg";
  btn_resources_on = new Image();
  btn_resources_on.src = theme_root + "images/btn_resources_on.jpg";
  
  btn_blog = new Image();
  btn_blog.src = theme_root + "images/btn_blog.jpg";
  btn_blog_on = new Image();
  btn_blog_on.src = theme_root + "images/btn_blog_on.jpg";
  
  btn_news = new Image();
  btn_news.src = theme_root + "images/btn_news.jpg";
  btn_news_on = new Image();
  btn_news_on.src = theme_root + "images/btn_news_on.jpg";
}

function switchOn(imgName) {
    if (document.images) {
      document[imgName].src = eval(imgName + "_on.src");
    }
}
  
function switchOff(imgName) {
    if (document.images) {
      document[imgName].src = eval(imgName + ".src");
    }
}