This will not be best answer to your query (there may be better solution), in iOS7 as application gets full access of screen, splash image size is 1024x768, where in iOS6 access starts after status bar so it's 1024x748.
If you will check option [Hide status bar during application launch] than for iOS 6 splash size will be 1024x768, so you don't need to add any extra image, check attached image to see that option in Target->general settings