function nav_roll(img_id,src_on,src_off){
	var src_on;
	var src_off;
	var img_id;
		function img_on(event){
			this.src= src_on;
		}
		function img_off(event){
			this.src= src_off;
		}
		document.getElementById(img_id).onmouseover = img_on;
		document.getElementById(img_id).onmouseout  = img_off;
	}