Multiple instances of query_posts

If you have 8 categories, you need 8 queries. To improve your code use WP_Query instead of query_posts:

<?php
$categories = get_categories();
if ( ! empty($categories) ) {
?>
<ul class="tabs">
<?php foreach ($categories as $c ) { ?>
<li>
  <a href="#tab<?php echo $c->term_id; ?>">
    <img src="<?php bloginfo('template_directory'); ?>/images/logo-<?php echo $c->slug; ?>.png" alt="<?php echo $c->name; ?>" />
  </a>
</li>
<?php } ?>
</ul> <!-- / .tabs -->
<?php } ?>

<?php if ( ! empty($categories) ) { foreach ($categories as $c ) { ?>
<div id="tab<?php echo $c->term_id; ?>">
<a href="#" class="left-button"><img src="prev.png" alt="Prev" /></a>
<div class="content">
<?php
$q = new WP_Query( array( 'posts_per_page' => -1, 'cat' => $c->term_id ) );
if ( $q->have_posts() ) : ?> <ul> <?php while( $q->have_posts() ) : $q->the_post();
?>
<li>
  <h3>
  <a href="https://wordpress.stackexchange.com/questions/108557/<?php the_permalink() ?>" rel="bookmark" title="Permanent Link to <?php the_title_attribute(); ?>">
  <?php the_title(); ?></a>
  </h3>
  <p><?php the_content(); ?></p>
</li>
<?php endwhile; ?> </ul> <?php endif; wp_reset_postdata(); ?>
</div><!-- / .content -->
<a href="#" class="right-button"><img src="next.png" alt="Next" /></a>
</div><!-- / #tab<?php echo $c->term_id; ?> -->
<?php } } ?>

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