SSL screenshot for New Blog Template


I set a G Forms form in SSL and there is screenshot image in http only and there is unsecure warning.

Well with the following how can I have a https url for the screensshot image:
if ( 'previewer' == $type ) {
$img = ( ! empty( $template['screenshot'] ) ) ? $template['screenshot'] : nbt_get_default_screenshot_url( $template['blog_id']);
$tplid = $template['name'];
$blog_url = get_site_url( $template['blog_id'], '', 'http' );

THis part of code is in blogtemplate.php

Thank you