如何将固定大小的照片从照片库带到UIImagePickerController？

Question

如何将照片库中固定大小的照片带到UIImagePickerController？

我目前的实施：

func pickPhotoFromGallery(){
    var picker = UIImagePickerController()
    picker.delegate = self
    picker.sourceType = .PhotoLibrary
    picker.mediaTypes = [kUTTypeImage as! String]
    self.presentViewController(picker, animated: true, completion: nil)
}