title: Update / Change the image of SKSpriteNode
body: <p>Is it possible to update/change the image of an <code>SKSpriteNode</code>?<br>
  The code below does not work:</p>
  
  <pre><code>var boss1 = SKSpriteNode(imageNamed: "boss1.png")
  boss1 = SKSpriteNode(imageNamed: "boss2.png")
  </code></pre>