Add Default Featured Image For Each Post In A Category http://wpsites.net/web-design/add-default-featured-image-for-each-post-in-a-category/

functions.php

function default_category_featured_image() {
global $post;
$featured_image_exists = has_post_thumbnail($post->ID);
              
if (!$featured_image_exists)  {
$attached_image = get_children( "post_parent=$post->ID&post_type=attachment&post_mime_type=image&numberposts=1" );

if ($attached_image) {
						  
foreach ($attached_image as $attachment_id => $attachment) {
set_post_thumbnail($post->ID, $attachment);
}}
else if ( in_category('54') ) {
set_post_thumbnail($post->ID, '40380');
}
else if ( in_category('55') ) {
set_post_thumbnail($post->ID, '40383');
}
else if ( in_category('56') ) {
set_post_thumbnail($post->ID, '40382');
}
else {
set_post_thumbnail($post->ID, '40381');
wp_reset_postdata();
							   }
							   
                           }
                        
      }
add_action('the_post', 'default_category_featured_image');

@braddalton Thanks a lot for pointing me in the right direction; I wasn't 100% sure that I had found the right tutorial.

@nikkoboy Here's the demo video showing what the code does https://www.youtube.com/watch?v=R7SL3l4SMYw