why is my vfl not working?

I have the following code which I am trying to implement a dynamic image added to a vertical scroll view. I would like to have its constraints so the image sets itself inside the borders of the scrollView.

However the results are that the image seems to remain in its original size (which is bigger then the scrollview so the image gets cropped)

Here is the code:

@IBOutlet weak var myScrollView: UIScrollView!
private let lettersModel:LettersModel = LettersModel();
private var imgs = [UIImageView]();

override func viewDidLoad() {
    super.viewDidLoad()

    myScrollView.backgroundColor = UIColor.brownColor()

    for var index=0; index<1; index++
    {

        let myImage:UIImage = UIImage(named: lettersModel.getLetterAt(index))!
        let myImageView:UIImageView = UIImageView()
        myImageView.image = myImage
        myImageView.contentMode = UIViewContentMode.ScaleAspectFit

        myScrollView.addSubview(myImageView)

        myImageView.translatesAutoresizingMaskIntoConstraints = false;

        myScrollView.addConstraints(NSLayoutConstraint.constraintsWithVisualFormat("H:|-[myImageView]-|", options: [], metrics: nil, views:["myImageView":myImageView] ))

        imgs.append(myImageView)

    }

}