In certain browsers - for example, Google Chrome - mixed http and https content is blocked by default. This can cause images to not be displayed.
At the moment, Mailchimp are serving the images exported from Mail Designer 365 over http. If your browser blocks mixed http and https content on web pages, this will explain why you are unable to see the images when you preview your templates via the Preview Mode.
We are aware of this issue and are currently working with the Mailchimp team to get it rectified.
Workaround
As a workaround, you can update your browser preferences to allow mixed content or switch to a different browser such as Safari, which by default allows mixed content.