Swift & SpriteKit: Full screen image
You need to add
scene.size = skView.bounds.size
or for Swift 5
scene.size = self.view.bounds.size
just before skView.presentScene(scene)
in your gameViewController file
This is what worked for me to stretch the background image to fit the whole screen in Swift 5.
background.size = CGSize (width: frame.maxX, height: frame.maxY)