Az mantık yürütelim ,

Eğer öne çıkan görsel yoksa, siz bir resmi öne çıakrılan görselmiş gibi davranmasını mı istiyorsunuz yazı içindeki ?

  function catch_that_image() {
   global $post, $posts;
   $first_img = '';
   ob_start();
   ob_end_clean();
   $output = preg_match_all('/<img.+src=[\'"]([^\'"]+)[\'"].*>/i', $post->post_content,      $matches);
   $first_img = $matches [1] [0];

   // no image found display default image instead
   if(empty($first_img)){
   $first_img = "/images/default.jpg";
   }
   return $first_img;
   }


function wpares__set_featured_image_on_save($post_id){
    $attachments = get_posts(array('numberposts' => '1', 'post_parent' => $post_id, 'post_type' => 'attachment', 'post_mime_type' => 'image', 'order' => 'ASC'));
    if(sizeof($attachments) > 0){
        set_post_thumbnail($post_id, $attachments[0]->ID);
    }else{
        // definning required libs. wp core.
        require_once(ABSPATH . 'wp-admin/includes/file.php');
        require_once(ABSPATH . 'wp-admin/includes/media.php');
        require_once(ABSPATH . 'wp-admin/includes/image.php');

        // load the images (core.)
        $img  = catch_that_image();
        if ("/images/default.jpg" != $img){
            $result = media_sideload_image($img, $post_id);
            $attachments = get_posts(array('numberposts' => '1', 'post_parent' => $post_id, 'post_type' => 'attachment', 'post_mime_type' => 'image', 'order' => 'ASC'));
            if(sizeof($attachments) > 0)
                set_post_thumbnail($post_id, $attachments[0]->ID);
        }else{
            //no images found
            return;
        }
    }
}



add_action( 'save_post', 'wpares__set_featured_image_on_save' );

function auto_set_post_image( $post_id ) {
    // verify if this is an auto save routine. 
      // If it is our form has not been submitted, so we dont want to do anything
      if ( defined( 'DOING_AUTOSAVE' ) && DOING_AUTOSAVE ) 
          return;

    // Check permissions
    if ( 'page' == $_POST['post_type'] ){
        if ( !current_user_can( 'edit_page', $post_id ) )
        return;
    }else{
        if ( !current_user_can( 'edit_post', $post_id ) )
            return;
    }

    // need to find and save the data

    //check if we have a post thumnail set already
    $attch = get_post_meta($post_id,"_thumbnail_id",true);
    if (empty($attch)){
        set_featured_image_on_save($post_id);
    }
}

Resmi alıp post_save olduğunda action eklemiş olduk burada.

Fonksiyonu da temanızın functions.php sine yapıştırın.

Afiyet olsun.