Automatically Add Featured Images to WordPress Posts

/**
 * Automatically set the first post image as featured image
 */
function custom_set_first_image_as_featured( $post_id ) {

    // Prevent autosave and revisions.
    if ( wp_is_post_autosave( $post_id ) || wp_is_post_revision( $post_id ) ) {
        return;
    }

    // Only process standard blog posts.
    if ( get_post_type( $post_id ) !== 'post' ) {
        return;
    }

    // Don't replace an existing featured image.
    if ( has_post_thumbnail( $post_id ) ) {
        return;
    }

    $post = get_post( $post_id );

    if ( ! $post || empty( $post->post_content ) ) {
        return;
    }

    preg_match(
        '/<img[^>]+wp-image-([0-9]+)/i',
        $post->post_content,
        $matches
    );

    if ( empty( $matches[1] ) ) {
        return;
    }

    $attachment_id = absint( $matches[1] );

    if ( $attachment_id ) {
        set_post_thumbnail( $post_id, $attachment_id );
    }
}

add_action( 'save_post', 'custom_set_first_image_as_featured', 20 );