My issue was similar.
This is what my experience has been on testing the IMG tag on gmail
(assuming most of the organization’s would have a dev qa and prod server.)
I had to send emails to customers on their personal email id’s and we could see that gmail would add something of its own like following to src attribute of img tag. Now when we were sending these images from our dev environment they would never render on gmail and we were always curious why?
so an image sent to my gmail id as following never worked for me
and our dev server we can’t render this image by hitting following URL on Chrome(or any browser).
now as long as the src has www on it worked all the time and we didnt had to add any other attributes.