Let users upload image(s) to the post from front end

wp_insert_attachment returns the resulting post_ID of attachment record created in posts table, so you will need to add these IDs (multiple) in the post meta table using update_post_meta as suggested in code below:

$attchmentIds = array();
if ($_FILES) {
    foreach ($_FILES as $file => $array) {
        $newupload = insert_attachment($file,$pid);
        $attchmentIds[] = $newupload;
    }
};
/*The following code will go after wp_insert_post call*/
update_post_meta($post_information, '_post_custom_attachments', $attchmentIds);

Now once this is done, when you want to display images on posts single page, you will need to get these attachment IDs from the post meta field and use wp_get_attachment_url function:

/*The_Loop*/
$attachmentIds = get_post_meta(get_the_ID(), '_post_custom_attachments');

foreach($attachmentIds as $attachmentId) {
    echo wp_get_attachment_url( $attachmentId );
    //outputs something like http://example.net/wp-content/uploads/filename
}

Reference Links:

http://codex.wordpress.org/Function_Reference/wp_insert_attachment
http://codex.wordpress.org/Function_Reference/wp_get_attachment_url

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