[flup]

Could be pretty simple though, in your header template look up the place where the header image is set:

<img src="$stylevar[titleimage]" border="0" alt="$vboptions[bbtitle]" />

And replace that with a random javascript image display code, like this one:

<script language="JavaScript">

function random_imglink(){
  var myimages=new Array()
  //specify random images below. You can have as many as you wish, upload these in your root images folder
  myimages[1]="img1.gif"
  myimages[2]="img2.gif"
  myimages[3]="img3.gif"

  var ry=Math.floor(Math.random()*myimages.length)

  if (ry==0)
     ry=1
     document.write('<img src="images/'+myimages[ry]+'" border=0>')
}

  random_imglink()
//-->
</script>

Guess that?ll do the job for you