This is a super simple and easy fix for the blurry image problem you may have noticed when reading emails on your retina capable device. Images and logos appear all blurry and nobody likes that.
The trick is to create a 2x version of your image, but to set the width and height values to the smaller version. For non-retina devices, you will get a higher res image that is resampled down, which should look good in most cases. For retina capable devices, the image is resampled up to its original higher resolution, but still remains crisp.
I recommend doing this for just important images in your email, like your logo and other icons that may have text in them, like share buttons, etc. Using this technique on complex photos may result in less than desirable anti aliasing effects.