// create new cookie-manager object
if (!cookieManager.getCookie("pageHeaderFile")) //if it doesn't exist, set cookie
   cookieManager.setCookie("pageHeaderFile",pageHeaderFile,"10800","seconds","www.bund-bawue.de","/");
else //otherwise read cookie
   pageHeaderFile = cookieManager.getCookie("pageHeaderFile");

// write css to document
document.write(   "<style type='text/css'>"+
                     "#header {"+
                        "background: transparent url(uploads/pics/"+pageHeaderFile+") no-repeat;"+
                     "};"+
                  "</style>");