A while back I wrote an article 3 Steps to Create an Interactive Email Using CSS that demonstrated a method to create rollover images in email using background images. However the old method requires the width of the image to be fixed because the background-size property is not uniformly supported by all email clients.

This article goes into how to create responsive rollover images.

Here’s a tool that generates the code for responsive rollover images.

You can also access an older version that uses background images here.