need to add attach thumbnail from my form

Its pretty simple actually. Here is the link for the set_thumbnail function reference codex. And here is the answer for the file upload.

First off all you have to attach the file to the post :

function attach_uploads($uploads,$post_id = 0){
$files = rearrange($uploads);
if($files[0]['name']==''){
    return false;   
}
foreach($files as $file){
    $upload_file = wp_handle_upload( $file, array('test_form' => false) );
    $attachment = array(
    'post_mime_type' => $upload_file['type'],
    'post_title' => preg_replace('/\.[^.]+$/', '', basename($upload_file['file'])),
    'post_content' => '',
    'post_status' => 'inherit'
);
    $attach_id = wp_insert_attachment( $attachment, $upload_file['file'], $post_id );
    $attach_array[] = $attach_id;
    require_once(ABSPATH . 'wp-admin/includes/image.php');
    $attach_data = wp_generate_attachment_metadata( $attach_id, $upload_file['file'] );
    wp_update_attachment_metadata( $attach_id, $attach_data );
}
return $attach_array;
}

More detail about the php file upload rearrangement : php.net.

And this is the function to set the uploaded file as post thumbnail. Put it in the corresponding wp_ajax function.

// put this one early, before any process
$files = $_FILES['profile-picture'];

// insert attachment, after you have the new post id of course
$attached_files = attach_uploads($files,$pid);

// set the first file as post thumbnail
// $attached_files[0] is for the first file

if($attached_files){
    set_post_thumbnail( $pid, $attached_files[0] ); 
}

Hope this help

ADDED :

function rearrange( $arr ){
    foreach( $arr as $key => $all ){
        foreach( $all as $i => $val ){
            $new[$i][$key] = $val;    
        }    
    }
    return $new;
}

Just put it in the functions.php