I'm using mailchimp for my newsletter and I will be checking with them about this too.
My feed is http://crowandrose.com/feed
When I look at the email in my computer and upon previewing in mailchimp, everything looks fine. But viewing the email in iphone (presumably other smart phones) the image shows an icon with a border that indicates the full size of the image.
Could the image be effecting the responsiveness of the email or does mailchimp (because it may not be responsive) effect the delivery of the image in smaller formats?