Source (mailchimp) : http://templates.mailchimp.com/development/responsive-email/fluid-images/

<img src="" alt="" height="" width="" class="emailImage" />
    <style >
        @media only screen and (max-width: 480px)
        {
            .emailImage
            {
                height:auto !important;
                max-width:600px !important;
                width: 100% !important;
            }
        }
    </style>

When the email is viewed on a screen that is 480px and smaller, the image has its height and width overridden, it also gets a max-width designated so that the image doesn’t look blown out proportion. As a rule of thumb, the max-width should be the same as the image’s original width. What we end up with, once the media query is triggered, is an image that fits on a small display.