WP Insert Post function – Insert Post Thumbnail

you need to first create the post and get the id:

$postit = array(
    'post_title' => $itemtitle,
    'post_content' => '',
    'post_status' => 'publish',
    'post_type' => 'items',
    'post_author' => $user_ID,
    'tags_input' => $the_post_id

);

 $the_post_idit = wp_insert_post( $postit);

Once you have the post id you can use

update_post_meta( $the_post_idit,'_thumbnail_id',$itemimage);

just make sure that $itemimage holds the attachment ID.