Set first attached image as Featured Image

<?php
/*
 * put the code in functions.php
 * visit any admin page in wordpress
 * delete the code
 */
function get_first_image($ID) {
    $args = array(
        'numberposts' => 1,
        'post_mime_type' => 'image',
       'post_parent' => $ID,
        'post_status' => null,
        'post_type' => 'attachment'
        );
        $attachments = get_children( $args );
        if ($attachments) {
            foreach($attachments as $att) return $att->ID;
        }
}
function make_featured_images() {
    // CHANGE THE POST TYPE AS NEEDED
    $posts = get_posts( array('numberposts' => -1, 'post_type' => 'post') );
    foreach ( $posts as $post ) {
        $featured = get_the_post_thumbnail( $post->ID, 'thumbnail', null );
        // WILL NOT RUN IF POST ALREADY HAS A FEATURED IMAGE
        if ( !$featured ) {
            $feat = get_first_image($post->ID);
            if($feat) set_post_thumbnail( $post->ID, $feat );
        }
    }
}
add_action('init', 'make_featured_images');