title: Showing a lock screen image using MPNowPlayingInfoCenter doesn't work
body: <p>I have the following code and is working for me  but it can only currently show artist and song :</p>
  
  <pre><code>let mpic = MPNowPlayingInfoCenter.defaultCenter()
  
  mpic.nowPlayingInfo = [MPMediaItemPropertyTitle:songs[currentAudioIndex].songName,
                          MPMediaItemPropertyArtist:songs[currentAudioIndex].artistName]
  </code></pre>
  
  <p>I tried also setting the picture with the following code but it isn't working :</p>
  
  <pre><code>mpic.nowPlayingInfo = [MPMediaItemPropertyTitle:songs[currentAudioIndex].songName,
                      MPMediaItemPropertyArtist:songs[currentAudioIndex].artistName,
                      MPMediaItemPropertyArtwork:songs[currentAudioIndex].songImage]
  </code></pre>
  
  <p>SongImage is of type  UIImage.</p>