Horizontal thumbnails for gallery

Hi i have been searching for hours to try and learn how to align my thumbnails horizontally on my website here

http://festivalnet.co.uk/test-list-with-gallery-2/

Can anyone let me know to achieve this?

This is my single.php where i call the gallery into the template

<?php the_category( $separator, $parents, $post_id ); ?>
<?php the_content(); ?>
<span class="about-us">Our Images</span> <br />
<?php echo do_shortcode("[gallery]"); ?>

Thanks
Al