Makes sure all posts have a featured image. Updates older posts and sets the first embedded image as the featured image. Also does the same for any new posts created.

wp-fix-featured-images.php

<?php
/**
 * Plugin Name: Fix Featured Images
 * Description: Makes sure all posts have a featured image. Updates older posts and sets the first embedded image as the featured image. Also does the same for any new posts created.
 * Plugin URI: http://briannadeleasa.com
 * Version: 0.1.0
 * Author: Brianna Deleasa
 * Author URI: http://briannadeleasa.com
 * Text Domain: fix-featured-images
 */


add_action( 'save_post', 'auto_set_featured' );
/**
 * When new posts are saved, if the user didn't specify a featured image,
 * this will pull the first embedded image and set it as the featured image.
 *
 * @since 1.0.0
 *
 * @param null $post
 * @return null
 */
function auto_set_featured( $post = NULL ) {

	$post = get_post( $post );

	// Get out if this isn't a post or if the post already has a thumbnail
	if ( ! $post instanceof WP_Post || has_post_thumbnail( $post->ID ) )
		return;

	$thumbnail = NULL;

	$images    = get_posts( array(
		'post_parent'    => $post->ID,
		'post_type'      => 'attachment',
		'post_status'    => 'inherit',
		'post_mime_type' => 'image',
		'posts_per_page' => 1
	) );

	// Get the first image and set it as the post thumbnail
	if ( is_array( $images ) && ! empty( $images ) )
		$thumbnail = reset( $images );

	if ( $thumbnail instanceof WP_Post )
		set_post_thumbnail( $post->ID, $thumbnail->ID );

}


/**
 * This is meant to only run once on older posts. It runs the function above
 * to set the first image as the featured image for posts without a featured
 * image.
 *
 * @since 1.0.0
 */
add_action( 'admin_init', function() {

	if ( (int) get_transient( 'bulk_auto_set_featured' ) > 0 )
		return;

	$posts = get_posts( 'posts_per_page=-1' ) ;

	if ( empty( $posts ) )
		return;

	array_walk( $posts, 'auto_set_featured' );

	set_transient( 'bulk_auto_set_featured', 1 );

} );