In SpriteKit a Sprite is represented by the SKSpriteNode class (which inherits from SKNode).
First of all create a new Xcode Project based on the SpriteKit template as described in Your First SpriteKit Game.
Now you can create a SKSpriteNode using an image loaded into the Assets.xcassets folder.
let spaceship = SKSpriteNode(imageNamed: "Spaceship")
Spaceshipis the name of the image item into the Assets.xcassets.
After the sprite has been created you can add it to your scene (or to any other node).
Open GameScene.swift, remove all its content and add the following
class GameScene: SKScene {
    override func didMoveToView(view: SKView) {
        let enemy = SKSpriteNode(imageNamed: "Spaceship")
        enemy.position = CGPoint(x:self.frame.midX, y:self.frame.midY)
        self.addChild(enemy)
    }
}
Now press CMD + R in Xcode to launch the Simulator.