var menu_img = document.getElementById("menu").getElementsByTagName("img");

for(i=1; i<=menu_img.length; i++){
	menu_img[i-1].idnum = i;
	menu_img[i-1].onmouseover = function(){
		  this.src = "images/menu-" + this.idnum + "b.jpg";
		}
	menu_img[i-1].onmouseout = function(){
		  this.src = "images/menu-" + this.idnum + ".jpg";
		}
}