Add the new png resource to the new resolution and add @ 2x to the file name. If you use IB, then it should be selected automatically. If you use -imageNamed: then do not include the .png extension in the image name.
For example: "TabBarIcon.png" will have the file " TabBarIcon@2x.png ", which has dual resolution and will be automatically downloaded by UIKit to iPhone 4.
see here for more information.