Create a link to full-sized image attachment?

I don’t know if TimThumb is really necessary. You could easily do that with internal WordPress functions.
Add this code to your functions.php:

<?php
add_theme_support( 'post-thumbnails' ); // this enable thumbnails
add_image_size( 'preview-thumb', 300, 185, true ); //this sets your default size
?>

Use the following code instead of your code above.

<div class="main">
  <div class="view view-first">
    <h4><a href="https://wordpress.stackexchange.com/questions/126373/<?php the_permalink() ?>" rel="bookmark"><?php the_title(); ?></a></h4>
    <div class="myimage">
      <?php
        if ( has_post_thumbnail())
        {
          $thumb = wp_get_attachment_image_src( get_post_thumbnail_id( get_the_ID() ), 'preview-thumb' );
          echo '<img src="' . $thumb[0] . '">';
        } else {
          echo 'No post thumbnail.';
        }
      ?>
    </div>
    <div class="mask">
      <?php
        if ( has_post_thumbnail())
        {
          $full = wp_get_attachment_image_src( get_post_thumbnail_id( get_the_ID() ), 'full' );
          echo '<a href="' . $full[0] . '">Some text.</a>';
        } else {
          echo 'No post thumbnail.';
        }
        the_content_limit($hp_mid_content_length,'');
      ?>
    </div>
  </div>
</div>

Then follow these steps to add a post thumbnail.