Retina throws up quite a few headaches with regards to scaling raster/bitmapped images so they look decent with no nasty jaggies.
For photos where you have higher resolution source material it’s not an issue but if you need for example to upscale a screenshot which was captured at 72dpi you don’t have many options.
This is where one of our old print tricks come into play.
In Fireworks or Photoshop under the “Image Size” dialog, set the size to 200% and from the “Resample” drop-down, choose “Nearest neighbor (preserve hard edges)”.
This will upscale the image, but tells all the pixels it makes up, to originate from their closest neighbour. Which in essence provides a closer approximation of your original, just with the visual equivalent of bigger pixels.
The end result once viewed @2x on retina will be a pixel perfect sharp screenshot.