var zeit = 3; //In Sekunden

var i = 0;
function change_pic()
{       
  i++;
  if (i > document.getElementById('count_loop_pics').value) i = 1;
  document.getElementById('loop_pic').src = 'fileadmin/layout/loop_img/' + i + '.jpg';   
  window.setTimeout ("change_pic()", (zeit * 1000));  
}