1，先把背景图片bg.jpg，bg@2x.jpg直接拖进Images.xcassets中

import SpriteKit

class GameScene: SKScene {
    //当切换到这个场景视图后
    override func didMoveToView(view: SKView) {
        //将Images.xcassets中的bg.jpg 来制作背景
        var bg = SKSpriteNode(imageNamed: "bg")
        //设置位置
        bg.position = CGPoint(x:CGRectGetMidX(self.frame), y:CGRectGetMidY(self.frame));
        self.addChild(bg)        
    }
    //响应屏幕点击的方法
    override func touchesBegan(touches: NSSet, withEvent event: UIEvent) {
        
    }
}