Let me quote a special behaviour
From Technical Note TN2244 Launching your iPhone Application in Landscape
Except for launch images used by the iPhone 6 Plus, asset catalogs assume that all iPhone launch images are for the portrait orientation.
Therefore
Launch images for iPhone apps are always sized to match the dimensions of the screen in portrait orientation. For applications that launch into landscape orientation, you should use your preferred graphics editing software to rotate the content of the launch image while keeping the image's size consistent with a portrait launch image (height > width).
if you don't do things correctly, you can end up with a blank/white screen
The result is a blank screen during launch as the system cannot find an appropriate launch image.
Unfortunately, the document was last updated 2 years ago.
2015-05-26 Updated for Xcode 6 and iOS 8.