When defining a editzable image in a custom template, Mailchimp strips the width-attribute from the tag: eg. <img mc:edit="myeditableimage" src="" width="600" ... />. The width-attribute is always stripped, thus making it impossible to use retina images in e-mails because these are displayed at their actual pixelsize in Outlook (for Windows) whatever class on inline style you add.

If not possible, does anyone a good alternative for Mailchimp that does mangle with the image-code?