For images. You must upload individual images with the correct size. For example, the Facebook timeline has a separate image for a custom icon (e.g. 25x25). and forever, when you have a photo, a sketch is created that has dimensions (310x160). And then when the user clicks on the image to view. It opens the original image. Whatever size he has. So, in your case. you need to upload two images. One for your non-retina and one for your retina. You can use the Flicker API for this.
Note. You can skip this and continue displaying the retina because retina devices are rarely used. Thus, everyone has a retina device and the application will work perfectly with your existing scenario.