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');

Thoughts on how to change this from in_category to use a taxonomy tag instead? Also, is there an easy way to say "if it's in more than one of those tag, use THIS image instead of the one previously defined"? Thanks!

Try is_tax( $taxonomy, $term );

@braddalton that's a super helpful code snippet!
I would like to tweak it so it forces to display a default featured image, EVEN if a post already has a featured image. Basically, I want to override ALL featured images site-wide and replace them ALL by default category images. How could this be achieved? Thanks!

@nikkoboy The code for that is in one of these tutorials https://wpsites.net/tag/featured-image/

@braddalton, thanks a lot! I'll have a look at it right away. Does the code also work for other types of taxonomies like for instance WP Job Manager's? They use job_listing_category instead of in_category.

@nikkoboy Yes the code will work for any/all taxonomies and custom post types.

@nikkoboy This is the code you need https://wpsites.net/web-design/display-the-same-featured-image-on-genesis-archive-pages-overriding-single-post-thumbnails/

@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