In the minute my slider (s3Slider JQuery) calls and shows the final 5 posts. It grabs the pictures from the custom area labelled 'thumb'.

I would like rather for that slider to simply call images which have something within the 'thumb' custom area. Is possible?

Current totally...

<?php
$my_query = new WP_Query('showposts=5');
while ($my_query->have_posts()) : $my_query->the_post();
?>
<li class="sliderImage"> <a href="<?php the_permalink() ?>" rel="bookmark"> <img class="featimg" src="<?php echo get_post_meta($post->ID, 'thumb', true) ?>" alt="<?php the_title(); ?>"  />
<span class="des"><h1><?php the_title(); ?></h1><?php the_excerpt(); ?></span>    
</a>
</li>
<?php endwhile; ?>

Thanks ahead of time.

You are likely to want to modify your Wordpress_Query towards the following (note this only works best for Wordpress >= 3.1):

$my_query = new WP_Query(
    array(
        'posts_per_page' => '5',
        'meta_query' => array(
            array(
                'key' => 'thumb',
                'value' => '',
                'compare' => '!='
            )
        )
    )
);

Obviously, if a person makes its way into something that is not a legitimate road to a picture (e.g. 'blahblah'), this it's still passed for your <img> in your <li> element, so you might want to perform some further checking/error handling when processing the input in the WordPress after sales.