Missing images in Mailchimp preview mode?
If Mailchimp's Preview Mode is not displaying your images, checkbox the Premium plugin setting "Disable Responsive Images".
Why isn't Mailchimp preview showing my images?
Mailchimp does not like the responsive image SRCSET code that WordPress adds to the RSS feed image when the website URL is not using HTTPS. Mailchimp does seem to use a proxy URL which uses HTTPS for the SRC image URL, but the SRCSET HTML seems to override that first image SRC URL, causing the broken image and insecure warning.
The image will usually work fine when sending the emails out, because it's not requiring HTTPS for images in Gmail or other email programs (yet).
You can ignore the preview issue, knowing the image will show in the Inbox, or update your website and feed URL to use HTTPS, or upgrade to the Premium version of the plugin, which provides superb support and has a checkbox for disabling the responsive images in the feed.
Is your website using secure URLs?
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. This does not impact the image showing in the emails actually sent. For overall security and web compatibility it is best to switch your website to HTTPS secure URLS. Need hosting help?
Some services don't like the responsive image output that WordPress includes with images (srcset), and so display broken image links. We've included a "Disable Responsive Images" checkbox to help with this. Simply check the box and it should resolve any display issues. Upgrade to Premium to access this feature.