title: Tableview scroll too slow loading images
body: <p>My issue is that in my app I load images in a table view.  When I scroll through the table view it is really slow and stops scrolling sometimes.  I looked online and saw that using dispatch would help.  I tried that with the code below and my app crashes saying that it is nil.  </p>
  
  <p><strong>Code:</strong></p>
  
  <pre><code>dispatch_async(dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_HIGH, 0)) {
  
                  let parseImage: PFFile = objectSong.valueForKey("picture") as PFFile
                  var pic: UIImage = UIImage(data: parseImage.getData())!
  
                      dispatch_async(dispatch_get_main_queue()) {
                          cell.imageView.image = pic   &lt;-- crash happens here
                      }
                  }
              }
  </code></pre>
  
  <p><strong>Previous code used to load table view cells:</strong></p>
  
  <pre><code>        let cell = tableView.dequeueReusableCellWithIdentifier("Cell", forIndexPath: indexPath) as TableViewCell
          let objectSong: PFObject = object as PFObject
  
          cell.songImage.image = UIImage(data: parseImage.getData())
  </code></pre>