iphone how to convert my app to retina display app

How can I convert my current application to retina display app and I've to make sure that the same app works in iphone 3 as well. If its just to update my imag开发者_Python百科es to high resolution, it will also work in iphone 3. then there will be no meaning for "retina display" app.

Retina Display version will use a higher resolution of an image (if it's supplied in the app). Refer to the Apple doc for details.