var num = 1;
var count = null;
var is = false; 
 
function set(num) {
         document.images["startbild"].src = "images/diashow/" + num + ".gif";
         }

function dia(anzahl) {
         num++;
         if (num > anzahl)
             num = 1;
             set(num);
         count = window.setTimeout("dia("+anzahl+")", 15000);
         is = true;
         }