The retina modifier is only useful when you request a smaller thumbnail than the original. So your original may be 1000x1000px, you use a 128x128 thumbnail on

The retina modifier is only useful when you request a smaller thumbnail than the original. So your original may be 1000x1000px, you use a 128x128 thumbnail on your regular (non-retina) site, and you serve 256x256 thumbnail to your retina users. If your original image is small then there is nothing to do.