function hightlight_img(_id){
	var img_doc= document.getElementById('nav').getElementsByTagName('img');
	var img_src= img_doc[_id].src;
	var ini_ext=img_src.substr(0, img_src.length-4)
	var added_ext= img_src.substr(img_src.length-4,img_src.length)
	
	img_doc[_id].src=ini_ext+"_sel"+added_ext;
	}