function preloadImg(imgObjName, imgURL) {
  if (document.images) {
    eval(imgObjName + " = new Image()");
    eval(imgObjName + ".src = imgURL");
  }
}

function displayImg(imgObjName, imgTagName) {
  if (document.images)
    document[imgTagName].src = eval(imgObjName + ".src");
}

preloadImg("img1off", "/img/but_nav_articles.gif");
preloadImg("img1on",  "/img/but_nav_articles2.gif");
preloadImg("img2off", "/img/but_nav_forums.gif");
preloadImg("img2on",  "/img/but_nav_forums2.gif");