Wordpress Featured Image to Post Attachment to Default Thumbnail Fallback

The code below is a 3-level fallback technique for WP image thumbnails. This code first loads the featured image if it's assigned to the post. If it is not set, the code will look for the first image attached to the post. Finally if there is no image associated with the post, a default 'no image' photo will be loaded, which should have been created and uploaded beforehand.

/* Function to process your thumbnail & image 
   Copy and paste the code below to your functions.php */

function get_attachment_picture(){
    global $post, $posts;
    $image_id = get_post_thumbnail_id();  //read featured image data for image url
    $attached_to_post = wp_get_attachment_image_src( $image_id, 'blog-grid' );
    $related_thumbnail =  $attached_to_post[0];                         

    if($related_thumbnail == ""):
        $attachments = get_children( array(
            'post_parent'    => get_the_ID(),
            'post_type'      => 'attachment',
            'numberposts'    => 1, 
            'post_status'    => 'inherit',
            'post_mime_type' => 'image',
            'order'          => 'ASC',
            'orderby'        => 'menu_order ASC'
            ) );
        if(!empty($attachments)):                                                   //check if there's an image attached or not
            foreach ( $attachments as $attachment_id => $attachment ) {
              if(wp_get_attachment_image($attachment_id) != ""):
                  $related_thumbnail = wp_get_attachment_url( $attachment_id );
              endif;                        
            }
        else:                                                                       // if no attachment 
            $first_img = '';
            ob_start();
            ob_end_clean();
            $output = preg_match_all('/<img.+src=[\'"]([^\'"]+)[\'"].*>/i', $post->post_content, $matches);
            $first_img = $matches [1] [0];

            if(!empty($first_img)):
                $related_thumbnail = $first_img;
            else:
                $related_thumbnail = bloginfo('template_directory')."/images/no-image.jpg";    //define default thumbnail, you can use full url here.
            endif;
        endif;   
    endif;  

    echo $related_thumbnail;
}   

The code above returns only an image URL. Call it as an image src attribute like below. For image resizing, a third-party resizing script is needed, like thumb.php or timthumb.php.

Copy and paste the code below in your template page like index.php, archive.php, single.php, etc.

<a href="<?php the_permalink(); ?>" title="<?php the_title_attribute(); ?>"><img src="<?php bloginfo('template_directory'); ?>/functions/thumb.php?src=<?php get_attachment_picture();?>&amp;h=300&amp;w=400&amp;zc=1" alt="<?php the_title_attribute(); ?>" class="attachment featured-image alignleft"></a>    <!-- with image resizing -->