Slideshowpro & WordPress

Hi I am using the code developed by theundersigned to build the xml doc for a number of slideshow pro gallerys, the script is below, I want to pull out a custom field from each post to be used as the gallery image, any help or advise on this would be greatly appreciated!

<?php
require('./wp-config.php');
header('Content-type: text/xml; charset=utf-8'); 

function ssp_fetch_images($postid, $size) {
    global $wpdb;
    return $wpdb->get_results("SELECT guid, post_title, post_content FROM $wpdb->posts WHERE post_parent = $postid AND post_mime_type = 'image/jpeg'");
} 

if(isset($_GET['cat'])) {
$cat = $_GET['cat'];
$query = "cat=$cat&showposts=1000";
} else {
$query = "showposts=1000";
} ?>
<gallery>
    <?php $gallery = new WP_Query($query);
    while ($gallery->have_posts()) : $gallery->the_post(); ?>
    <album title="<?php echo htmlspecialchars($post->post_title); ?>" description="<?php echo htmlspecialchars($post->post_content); ?>">
        <?php $images = ssp_fetch_images($post->ID);
        foreach($images as $image) { ?>
        <img src="<?php echo $image->guid; ?>" tn="<?php echo str_replace(".jpg", ".thumbnail.jpg", $image->guid); ?>" title="<?php echo htmlspecialchars($image->post_title); ?>" caption="<?php echo htmlspecialchars($image->post_content); ?>" />
        <?php } ?>
    </album>
    <?php endwhile; ?>
</gallery>