Mailchimp Preview Showing Broken Images? Disable Responsive Images.
If your website is not secured using HTTPS (it starts with http:// and not https://) then the insecure image breaks the Preview Mode display, but if your website loads over HTTPS, then your image is secure and Mailchimp's Preview Mode works fine.
Also note that some services don't like the responsive image output that WordPress includes with images (srcset), and so display broken image links.
Mailchimp's campaign preview is one that may have issues with responsive images, and so we've included a Disable Responsive Images checkbox to help with this; simply check the box, and it should resolve any display issues.