How to output placeholder image if no featured image set?

I am using the Multipostthumbnails plugin – https://wordpress.org/plugins/multiple-post-thumbnails/

And it has a certain naming convention to it. I know the usual way to output a placeholder if no featured image set, but not with their code?

I obviously want something like –

if ( has_post_thumbnail($post->ID) ){   
$image = wp_get_attachment_image_src(get_post_thumbnail_id(), 'news-large');
$image = $image[0];
echo '<img src="'.$image.'" alt="" />';
} else {  
$image = get_template_directory_uri() .'/assets/img/placeholders/placeholder.png'; 
echo '<img src="'.$image.'" alt="" />';
}

But their existing code to show a featured image is –

if (class_exists( 'MultiPostThumbnails' )) : MultiPostThumbnails::the_post_thumbnail(get_post_type(), 'header-image', NULL, 'header-image-full', array('class' => "custom-header-image") ); endif;

Just cannot see how to adjust this to fit my code at the top?

For Blog Pages –

Standard Code –

if (class_exists( 'MultiPostThumbnails' )) : MultiPostThumbnails::the_post_thumbnail( 'page', 'header-image', get_option('page_for_posts'), 'header-image-full', array('class' => "custom-header-image") ); endif;

My attempt to try and show placeholder –

if ( class_exists( 'MultiPostThumbnails' ) ) { 
                        MultiPostThumbnails::the_post_thumbnail( 'page', 'header-image',     get_option('page_for_posts'),
                            'header-image-full', 
                            array('class' => "custom-header-image") ); 

                        } else {  
                            $image = get_template_directory_uri() .'/assets/img/placeholders/header_placeholder.png'; 
                            echo '<img src="'.$image.'" alt="" />';
                        }

Many thanks

I think that you’re adding the code inside the loop of index.php or blog template file. You might be try this kind of things:

if ( class_exists( 'MultiPostThumbnails' ) && 
     ( MultiPostThumbnails::has_post_thumbnail( get_post_type(), 'header-image' , get_the_ID() ) ) ) { 
       MultiPostThumbnails::the_post_thumbnail(get_post_type(), 'header-image', NULL, 
                          'header-image-full', 
                           array('class' => "custom-header-image") ); 

} else {  
  $image = get_template_directory_uri() .'/assets/img/placeholders/placeholder.png'; 
  echo '<img src="'.$image.'" alt="" />';
}