InfoWindow Image Size

Hi Guys

Im using a custom infoWindow based on the function from Vikas in this post:
https://wpgeodirectory.com/support/topic/change-layout-of-google-info-box/

Ive copied and changed the original infoWindow code but I want to make the preview image the infoWindow loads a thumbnail so that it does not create a heavy load on the site.

Looking at the original code below used to load the image, how would I make it load the WP thumbnail or a custom image size instead?

$post_images = array();
if(!empty($postinfo_obj->post_images))
$post_images = explode(",",$postinfo_obj->post_images);

Thanks!