Add this code to the top of index.php under the includes.

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;

?>
Then you would use :

Code:
<TABLE cellSpacing=0  border="0"  cellPadding=0 width="100%" background="<?php $image_folder/$image_name?>">
Instead of:

Code:
<TABLE cellSpacing=0  border="0"  cellPadding=0 width="100%" background="../modimg/fondhaut.jpg">
Hope this helps