var images = document.images;
for (var i = 0; i < images.length; i++) {
    var img = images[i];
    var oversrc = img.getAttribute? img.getAttribute('oversrc') : 
    	img.oversrc? img.oversrc : null;
    	
    if (oversrc) {
        img.overImage = new Image();
        img.overImage.src = oversrc;
        img.onmouseover = function(e) {
            if (!this.normalImage) {
                this.normalImage = new Image();
                this.normalImage.src = this.src;
            }
            this.src = this.overImage.src;
        }
        img.onmouseout = function(e) {
        	if (this.normalImage) this.src = this.normalImage.src;
        }
    }
}