Put this code in a file called random.php:
Code:
<?php

// Name your images 1.jpg, 2.jpg etc.


// Change this to the total number of images in the folder
$total = "11";

// Change to the type of files to use eg. .jpg or .gif
$file_type = ".jpg";

// Change to the location of the folder containing the images
$image_folder = "images/random";

// You do not need to edit below this line

$start = "1";

$random = mt_rand($start, $total);

$image_name = $random . $file_type;

echo "$image_folder/$image_name";

?>
Then use this code in your header.tpl:

Code:
<TABLE cellSpacing=0  border="0"  cellPadding=0 width="100%" background="{include file="../random.php"}">
That should work :)