I am planning an app for iphone 3g, 3gs and iphone 4. I have a 57*57 image for 3g and 3gs. When I checked documentation apple suggests us to use a 114*114 image for iphone 4. So how can I support both the images with the same build.
Call the iPhone 4 icon [email protected]
http://developer.apple.com/library/ios/#qa/qa2010/qa1686.html
You have to list all your icons in your Application's plist, and name the ones for iPhone 4 "[email protected]" (add '@2x' to your file's name).
If you place the original icon and an icon with the same name but add @2x to the end of the name in your resources folder the correct one will be used automaticly.
Use the CFBundleIconFiles property in your Info.plist and supply two icons, you name the high-resolution one with an "@2x" suffix so the iOS knows it's the high-resolution icon to use on iPhone 4. You can also specify iPad icons (for apps that run on iPhone and iPad) using the same property. This is documented here: http://developer.apple.com/library/ios/#documentation/iPhone/Conceptual/iPhoneOSProgrammingGuide/BuildTimeConfiguration/BuildTimeConfiguration.html
The collection of different possible icon sizes for iOS is getting pretty long now. But all you need to do is to list all of the files in your app's Info.plist in the CFBundleIconFiles
key, and the OS will do the work of picking the correct size.