What screenshots are actually required for submission?
When you optimized you app to iPhone 6 and 6 Plus you have to add new image and icon sizes in your assets. If you select Images.xcassets in your project and then select your AppIcon and hit ⌥+⌘+0 you must see something like that:



If you have selected iOS 8.0 and Later Sized you will have to provide screenshots for iPhone 6 and 6 Plus, in other case, it won't be necessary.