It looks like you'll need to use the Launch Screen storyboard or xib file to support launch screens specific for iPad Pro.
Per Apple, you should be able to use both the launch image assets to support pre-iOS8, and the Launch Screen file for iOS8+. From the Apple App Distribution Guide :
"For iOS 7 deployment targets, you can supply both a launch screen file and launch images. In iOS 8, the launch screen file is used, and in iOS 7, the launch images are used."
also these docs state:
"If you also need to support earlier versions of iOS, you can continue to supply static launch images in addition to a launch file."
However, in my case even though I have a LaunchScreen.storyboard file, it isn't getting used when I have launch images in an asset catalog. i.e.: I always see the asset catalog launch images instead of the launch screen. I verified the Launch Screen file is set in the info.plist and that it has the "Use as Launch Screen" flag set. I'm also using size classes and auto layout.