A few months ago Gmail switched the webmail interface to HTTPS for all of their users.
This is good news for gmail users and I think all webmail services should do this. Now gmail users can read their email securely without having to install all kind of hacks in Firefox.
The problem is that now if you send email with remote images in the html content and you don’t host them on a HTTPS server with a valid certificate ( signed by a recognized certificate authority ), the recipient’s browser is going to complain when they try to view your messages and this may scare your subscribers and lead to lower click rates for gmail users
To fix this all you have to do is serve the images over a SSL ( HTTPS ) connection. But to do this you need to buy a SSL certificate and since we already have one we thought we should let you use it so now you can host your images on our SSL enabled web server.
To do this you have to use the new “Upload” feature we just enabled a few days ago. All you have to do is design the html message in your prefered editor,use relative URLs for the images then create a zip file with the html message and and the images and upload it. See more details on the wiki page