When I try creating a button and setting a background image in Swift:

```
let button   = UIButton.buttonWithType(UIButtonType.System) as UIButton
    button.frame = CGRectMake(100, 100, 100, 100)
    button.setImage(IMAGE, forState: UIControlState.Normal)
    button.addTarget(self, action: "btnTouched:", forControlEvents: UIControlEvents.TouchUpInside)

    self.view.addSubview(button)

```

I always get an error: "**Cannot convert the expression's type 'Void' to type 'UIImage'**".