Mailchimp Preview Showing Broken Images? Disable Responsive Images.
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 we know of. While we hope they fix this soon, for now, we've included a Disable Responsive Images checkbox to help with this; simply check the box, and it should resolve the issue.
We've also noticed that if your website is not secured using HTTPS via TLS/SSL then Mailchimp's method of proxying the image breaks when responsive images is used, but if your website loads over https, then responsive images does not affect it and Mailchimp's Preview Mode works fine, and you don't need to disable responsive images.