Set a single featured image to all the NanoSupport Knowledgebase posts on activation. Before activating the plugin set the default post_thumbnail_id on line #19

bulk-fi-kb.php

<?php
/**
 * Plugin Name: Bulk Featured Image (KB)
 * Description: Set a single featured image to all the NanoSupport Knowledgebase posts on activation. Before activating the plugin set the default post_thumbnail_id on line #19.
 * Plugin URI: http://nanodesignsbd.com/
 * Author: Mayeenul Islam
 * Author URI: http://nanodesignsbd.com/
 * Version: 1.0.0
 * License: GPL2
 */

/**
 * Add designated Same Featured Image to KB articles
 * on Activation of the plugin.
 */
register_activation_hook( __FILE__, function() {

	// Attachment ID of a common image for all the KB posts.
	$thumbnail_id = 12; // <----------------- SET YOURS HERE

	$kbposts = get_posts( array(
							'post_type'   => 'nanodoc', // KB posts only
							'numberposts' => '-1' // get all of 'em
						) );

	// Go through all of 'em one by one.
	foreach( $kbposts as $kbpost ) {
		// If it already set one, don't do anyting.
		if( ! has_post_thumbnail( $kbpost->ID ) ) {

			// it has no featured image, set our default one to this one.
			set_post_thumbnail( $kbpost->ID, $thumbnail_id );
		}
	}

});


/**
 * FALLBACK :: IF ANYTHING GO WRONG
 * Remove all the Featured Images from KB articles
 * on DeActivation of the plugin.
 */
/*register_deactivation_hook( __FILE__, function() {

	$kbposts = get_posts( array(
							'post_type'   => 'nanodoc', // KB posts only
							'numberposts' => '-1' // get all of 'em
						) );

	// Go through all of 'em one by one.
	foreach( $kbposts as $kbpost ) {
		// If it already set one, don't do anyting.
		if( has_post_thumbnail( $kbpost->ID ) ) {

			// Delete featured images.
			delete_post_thumbnail( $kbpost->ID );
		}
	}

});*/