How to add image in submit post form for my guest uploads

Hi guys, can you help me on this. i created a submit form post. I was able to upload an image, the thing is, it is not attaching to the post submitted. here’s my code.

<?php /* Template Name: Submit Post Form */ get_header();

if( 'POST' == $_SERVER['REQUEST_METHOD'] && !empty( $_POST['action'] ) &&  $_POST['action'] == "new_post") {

// Do some minor form validation to make sure there is content
if (isset ($_POST['title'])) {
	$title =  $_POST['title'];
} else {
	echo 'Please enter a  title';
}
if (isset ($_POST['description'])) {
	$description = $_POST['description'];
} else {
	echo 'Please enter the content';
}
$tags = $_POST['post_tags'];

// Add the content of the form to $post as an array
$new_post = array(
	'post_title'    => $title,
	'post_content'  => $description,
	'post_category' => array($_POST['cat']),  // Usable for custom taxonomies too
	'tags_input'    => array($tags),
	'post_status'   => 'draft',           // Choose: publish, preview, future, draft, etc.
	'post_type' => 'great_images'  //'post',page' or use a custom post type if you want to
);

            if (!function_exists('wp_generate_attachment_metadata')){
                require_once(ABSPATH . "wp-admin" . '/includes/image.php');
                require_once(ABSPATH . "wp-admin" . '/includes/file.php');
                require_once(ABSPATH . "wp-admin" . '/includes/media.php');
            }
             if ($_FILES) {
                foreach ($_FILES as $file => $array) {
                    if ($_FILES[$file]['error'] !== UPLOAD_ERR_OK) {
                        return "upload error : " . $_FILES[$file]['error'];
                    }
                    $attach_id = media_handle_upload( $file, $new_post );
                }   
            }
            if ($attach_id > 0){
                //and if you want to set that image as Post  then use:
                set_post_thumbnail($new_post,'_thumbnail_id',$attach_id);
            }

//save the new post
$pid = wp_insert_post($new_post);
wp_redirect(get_permalink($pid)); exit;
//insert taxonomies
} ?>

<div id="wrap">
	<div id="header">
    	<?php get_template_part('logo');
        get_template_part('nav'); ?>
    </div>
    <div id="content">
    	<div id="main">
            <?php get_template_part('searches'); ?>
            <div class="single-post-item">
                <h1>Ask a Question</h1>
                <div class="post-meta">Fill out the fields below, all of them are required!</div>
                <div class="inner-content">
                	<!-- New Post Form -->
                    <div id="postbox">
                    	<form id="new_post" name="new_post" method="post" action="" enctype="multipart/form-data">
                            <!-- post name -->
                            <p><label for="title">Title</label><br />
                            <input type="text" id="title" value="" tabindex="1" size="20" name="title" />
                            </p>
                        
                            <!-- post Category -->
                            <p><label for="Category">Category:</label><br />
                            <p><?php wp_dropdown_categories( 'tab_index=3&taxonomy=category' ); ?></p>
                            
                            
                            <!-- post Content -->
                            <p><label for="description">Content</label><br />
                            <textarea id="description" tabindex="4" name="description" cols="50" rows="6"></textarea>
                            </p>
                            
                            <!-- post tags -->
                            <p><label for="post_tags">Tags:</label>
                            <input type="text" value="" tabindex="5" size="16" name="post_tags" id="post_tags" /></p>

                            <input type="file" name="thumbnail" id="thumbnail">
                            <p align="right"><input type="submit" value="Publish" tabindex="6" id="submit" name="submit" /></p>
                            
                            <input type="hidden" name="action" value="new_post" />
                            <?php wp_nonce_field( 'new-post' ); ?>
                    	</form>
                        
                    </div>
                </div>
            </div>
        </div>
        <?php get_sidebar(); ?>
    </div>
</div>
<?php get_footer(); ?>