On Click random header image

I know I’m probably being a thicky but I can’t figure out how to make my (already random) header image randomise also when clicked.

here is my header code for it:

<?php
$randomnumber = rand(1,3);
?>
<div id="header">
	<div id="headerimg">
<img id="blog-title" src="http://.../sszheader<?php print $randomnumber; ?>.jpg" alt="GO" />
	</div>

I need this same image to also be a link to randomise itself using the predefined ‘sszheader(randomnumber)’ variable.

thanks
jonny