Add a custom default avatar to WordPress

/**
 * This can be used to apply a custom default avatar to your WordPress
 * install.  Change the $custom_avatar value below to the specific URL 
 * of the custom image you'd like to use.
 * 
 * @uses avatar_defaults
 * @see  https://developer.wordpress.org/reference/hooks/avatar_defaults/
 * 
 * Note: This is for the DEFAULT avatar you set in Settings > Discussion
 * for when the user does not have an avatar. Using a custom image
 * for a specific user's avatar is something different.
 */
add_filter( 'avatar_defaults', function( $avatar_defaults ) {
    $custom_avatar = 'https://yourdomain.com/wp-content/uploads/path/to/file/custom_avatar_image.png';
    $avatar_defaults[ $custom_avatar ] = "Custom Avatar";
    return $avatar_defaults;
});