<html>
<body>
<script language="JavaScript" type="text/javascript">
function change_pic()
{
new_img = "/img/pic2.gif";
document.getElementById("pic2change").src = new_img;
}
setTimeout("change_pic()", 5000); // смена через 5 секунд
</script>
<img src="/img/pic1.gif" id="pic2change">
</body>
</html>