The person sending the emails creates a dynamic web page. It can use all of the latest and greatest web technologies. (AJAX, WebSockets, CSS3, SVG, etc) Then they create a Web Crop in our dashboard, and we give them an HTML embed code.
When a user opens the email, it points to an image on our servers. At that point, we load the dynamic web page, take a screenshot, crop it, and render it in your browser. But at that point instead of closing the connection, we leave it open. Every second, we repeat the process, replacing the old image with the new one.