To get preview images with HTTPS please follow these steps:
1. Do an API call with the following query: /v2/assets/[embed_code]/primary_preview_image 2. As result of the above call you will get a list of URL and different sizes for the preview image you have selected on Backlot. 3. Each size has a HTTP and HTTPS URL. Look for the "url_ssl" value, that one will be HTTPS.