Rettifico, ci sono riuscito. Con un altro script però... ve lo riporto, magari può servire a qualcuno.

<script language="Javascript">
<!--
function image() {
};

image = new image();
number = 0;
// imageArray

image[number++] = "<img src='random/imm001.jpg'>"
image[number++] = "<img src='random/imm002.jpg'>"
image[number++] = "<img src='random/imm003.jpg'>"
image[number++] = "<img src='random/imm004.jpg'>"
image[number++] = "<img src='random/imm005.jpg'>"
image[number++] = "<img src='random/imm006.jpg'>"
image[number++] = "<img src='random/imm007.jpg'>"
image[number++] = "<img src='random/imm008.jpg'>"
image[number++] = "<img src='random/imm009.jpg'>"
image[number++] = "<img src='random/imm010.jpg'>"
image[number++] = "<img src='random/imm011.jpg'>"

increment = Math.floor(Math.random() * number);
//-->
</script>

<script language="JavaScript">
document.write(image[increment]);
</script>