Here are the steps to create a crisp logo for Retina displays
1. Take your current logo and double the resolution (144 dpi). The document size width and height do not change but the pixel dimension will change to double it's normal size(Leave as is when doubled).
2. Rename the logo with @2x after the original name and before the dot extension: mylogo.png will be email@example.com
3. Upload the new logo to the LAYOUT>LOGO section under Desktop Logo File.
4. Upload different filenames with @2x after the original name and before the dot extension in different sizes, to customize the logos for the TABLET and MOBILE areas under DESIGNS>MOBILE/TABLET. Otherwise the TABLET & MOBILE logos will default to use the Desktop Logo File
TIP: You can even use .svg for logos so you don't have to worry about retina vs regular screen. Upload those without the @2x (mylogo.svg).