Set Default Featured Image and Thumbnail for New Posts

Hello,

We use the same Featured Image for all of our new posts, and I am trying to edit my template code so that it automatically sets the featured image. I know the URL of the image I want to use, but could someone please help me with editing the code?

Code in Blog Page Template:

<?php $post_image_size = of_get_option('post_image_size'); ?>
      <?php if($post_image_size=='' || $post_image_size=='normal'){ ?>
        <?php if(has_post_thumbnail()) {
          echo '<a href="'; the_permalink(); echo '">';
          echo '<div class="featured-thumbnail"><div class="img-wrap">'; the_post_thumbnail(); echo '</div></div>';
          echo '</a>';
          }
        ?>
      <?php } else { ?>
        <?php if(has_post_thumbnail()) {
          echo '<a href="'; the_permalink(); echo '">';
          echo '<div class="featured-thumbnail large"><div class="img-wrap"><div class="f-thumb-wrap">'; the_post_thumbnail('post-thumbnail-xl'); echo '</div></div></div>';
          echo '</a>';
          }
        ?>
      <?php } ?>

Code in Single Post:

<?php $single_image_size = of_get_option('single_image_size'); ?>
				<?php if($single_image_size=='' || $single_image_size=='normal'){ ?>
          <?php if(has_post_thumbnail()) {
            echo '<div class="featured-thumbnail"><div class="img-wrap">'; the_post_thumbnail(); echo '</div></div>';
            }
          ?>
        <?php } else { ?>
          <?php if(has_post_thumbnail()) {
            echo '<div class="featured-thumbnail large"><div class="img-wrap"><div class="f-thumb-wrap">'; the_post_thumbnail('post-thumbnail-xl'); echo '</div></div></div>';
            }
          ?>
        <?php } ?>

Any help would be much appreciated.