You will not be able to do what you are trying to do by naming alone. If you don't want to use the asset catalog (which totally solves the problem without effort, so why would you not use it????), you will have to set up the Info.plist yourself, as Apple explains here:
In iOS 7, an app can use a different launch image depending on which version of iOS it’s running in. To provide different launch images, add the UILaunchImages key to the Info.plist file and use a dictionary to describe each launch image. To learn more about handling launch images, see “App Launch (Default) Images” in iOS App Programming Guide.