Prova questo ....

codice:
<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN">

<html>
<head>
<title>Untitled</title>
<script language="JavaScript" type="text/javascript">
<!--
var a = new Array();
a[0]       = "http://localhost/images/Image1.gif";
a[1]       = "http://localhost/images/Image2.gif";
a[2]       = "http://localhost/images/Image3.gif";
a[3]       = "http://localhost/images/Image4.gif";
imageIndex = 0;

function avanti() {
 image(1);
}
function indietro() {
 image(-1);
}

function image(i) {
 imageIndex = imageIndex + i;
 if (imageIndex < 0) {imageIndex=0}
 if (imageIndex >= a.length) {imageIndex=a.length-1}
 document.getElementById('iFrame').src = a[imageIndex];
}
//-->
</script>

</head>
<body>
<iframe id="iFrame" src="" scrolling="no"></iframe>


<input type="button" value="Indietro" onclick="indietro()">
<input type="button" value="Avanti" onclick="avanti()">
<script language="JavaScript" type="text/javascript">
<!--
//------ Visualizzare Prima immagine 
image(0);
//-->
</script>

</body>
</html>