var picture_number = 1;

setInterval('change_picture()', 4000);


function picture_url() {
   
   var url = picture_number.toString();
   if (url.length == 1) {
	   url = '0' + url;
   }
   
   if (url) {
	   url = 'url("./pictures/' + url + '.jpg")';  
   }
   
   return url;
}

function change_picture() {
	
    picture_number += 1;
    if (picture_number == 12) {
       picture_number = 1;
    }
	
    var url = picture_url(picture_number);
    var picture_box = document.getElementById('picture_box');
    picture_box.style.background = url;
}