Customizing the html of the uploaded images

<?php get_header(); ?>

          <?php

          $header_image = wp_get_attachment_image_src( get_post_thumbnail_id( $post->ID ), 'full' );       

          ?>

          <!-- Content Wrapper -->
          <div class="content-wrapper large-12 columns clearfix" data-equalizer-watch>
          <?php if ( have_posts() ) : while ( have_posts() ) : the_post(); ?>  

            <?php if (get_the_post_thumbnail()): ?>
              <div id="content-header" style="background-image: url('<?php echo $header_image[0]; ?>');">
                <h1><?php the_title(); ?></h1>  
              </div>
            <?php endif; ?>  

            <div id="content">

              <?php if (get_the_post_thumbnail() == TRUE ): ?>

                <h1 class="hide-title"><?php the_title(); ?></h1>

              <?php else: ?>

                <h1><?php the_title(); ?></h1>  

              <?php endif; ?>

              <?php the_field('description'); ?>  

            </div>          
            <!-- End Content -->

            <aside id="sidebar_right">
              <h4>Sidebar</h4>


              <img src="<?php $image_url; ?>">             

              <ul class="content_gallery">

                <?php

                 $args = array(
                   'post_type' => 'attachment',
                   'numberposts' => -1,
                   'post_status' => null,
                   'post_parent' => $post->ID
                  );

                  $attachments = get_posts( $args );
                     if ( $attachments ) {
                        foreach ( $attachments as $attachment ) {
                          $image_src = wp_get_attachment_url( $attachment->ID);
                           echo '<li>';
                           echo '<a class="th" href="' . $image_src . '">';
                           echo '<img src="' . $image_src . '">';
                           echo '</a>';
                           echo '</li>';
                          }
                     }

                ?>              

              </ul>
            </aside>                         
            <?php endwhile; endif; ?> 
<?php get_footer(); ?>