[assassingod]

This is totally untested, but try add this to the phpinclude_start template:

$numimg = '4'; // number of images
$rimg = rand(1,$numimg);

$randomimage = $rimg;

Then putting

<img src="$vboptions[bburl]/$stylevar[imgdir]/$randomimg.gif" alt="Random Image">

In the header template. Make sure all the images have the same extensions, and each file is called 1,2,3 etc...