XF 1.5 how to get more content on CDN server?

[sross]

HI, so I am up and running on stackpath CDN (formerly maxcdn). It's running well. Basic config is:

general style settings image path
https://blahblah.stackpathcdn.com/styles/default

config.php

$config ['externalDataUrl'] = 'https://blahblah.stackpathcdn.com/data';
$config ['javaScriptUrl'] = 'https://blahblah.stackpathcdn.com/js';

I'm not 100% sure how it adds assets such as thumbnails but not the actual images files. I see config external data url is /data well, my gallery thumbs are in /data and gallery images are in /data

Does anyone have tips for offloading more of /data to the CDN? Thanks