非同期処理での実行される順番について

AWSののdynamoDBを用いて、loadUser()データベースに上がってある画像のURLを持ってきて、downloadImage()で画像をダウンロードして、addImage()でscrollViewに追加するようなことをしています。
しかし、downloadImage()内でダウンロードされた順番(loadCounter)を出力してみると
0
2
1
などのように出力されて、順番通りに出力されませんでした。
非同期処理で順番通りにダウンロードするにはどこを変更したらいいでしょうか。お願いします。

import UIKit
import AWSCore
import AWSS3
import AWSDynamoDB

class LoadViewController: UIViewController, UINavigationControllerDelegate,UIScrollViewDelegate {


    var loadImages:[UIImage] = []
    var loadItems:[DDBTableRow] = []

    var userId:String?
    var userName:String?
    var pagniatedOutput: AWSDynamoDBPaginatedOutput?

    @IBOutlet weak var scrollView: UIScrollView!

    let defaults = NSUserDefaults.standardUserDefaults()

    override func viewDidLoad() {
        super.viewDidLoad()

        self.scrollView.delegate = self

        self.userName = defaults.stringForKey("userName")
        self.userId = defaults.stringForKey("userId")


        loadUser()

    }
    override func didReceiveMemoryWarning() {
        super.didReceiveMemoryWarning()
        // Dispose of any resources that can be recreated.
    }


    func addImage(image:UIImage)
    {
        let sheight = self.scrollView.frame.height
        let swidth = self.scrollView.frame.width

        loadImages.append(image)
        let myImageView = UIImageView()
        myImageView.frame = CGRectMake(swidth * CGFloat(loadImages.count-1), 0, swidth, sheight)
        myImageView.image = image
        myImageView.contentMode = .ScaleAspectFit
        scrollView.addSubview(myImageView)
        scrollView.contentSize = CGSizeMake(swidth * CGFloat(loadImages.count), sheight)

    }



    func loadUser()
    {
        let dynamoDBObjectMapper = AWSDynamoDBObjectMapper.defaultDynamoDBObjectMapper()

        let queryExpression = AWSDynamoDBQueryExpression()
        queryExpression.hashKeyValues = userId
        queryExpression.scanIndexForward = false
        var loadCounter:Int = 0
        dynamoDBObjectMapper .query(DDBTableRow.self, expression: queryExpression) .continueWithExecutor(AWSExecutor.mainThreadExecutor(), withBlock: { (task:AWSTask!) -> AnyObject! in
            if (task.error != nil) {
                print("Error: \(task.error)")

            } else {
                if (task.result != nil) {
                    self.pagniatedOutput = task.result as? AWSDynamoDBPaginatedOutput

                    for item in (self.pagniatedOutput?.items)!
                    {
                        let tableRow = item as! DDBTableRow

                        print(tableRow.Date)
                        self.downloadImage(tableRow.ImageURL!,loadCounter: loadCounter)
                        self.loadItems.append(tableRow)
                        loadCounter += 1

                    }
                }
                self.performSegueWithIdentifier("unwindToMainSegue", sender: self)
            }
            return nil
        })
    }

    func downloadImage(S3DownloadKeyName: String,loadCounter:Int){

        var completionHandler: AWSS3TransferUtilityDownloadCompletionHandlerBlock?

        let queue:dispatch_queue_t = dispatch_queue_create("com.gologo13.gcd", DISPATCH_QUEUE_SERIAL)

        let expression = AWSS3TransferUtilityDownloadExpression()
                expression.downloadProgress = {(task: AWSS3TransferUtilityTask, bytesSent: Int64, totalBytesSent: Int64, totalBytesExpectedToSend: Int64) in
                    dispatch_async(dispatch_get_main_queue(), {
                        let progress = Float(totalBytesSent) / Float(totalBytesExpectedToSend)
                        NSLog("Progress is: %f",progress)
                    })
                }


        completionHandler = { (task, location, data, error) -> Void in
            dispatch_async(dispatch_get_main_queue(), {
                if ((error) != nil){
                    NSLog("Failed with error")
                    NSLog("Error: %@",error!)
                }
                else{
                    print(Success)
                    self.addImage(UIImage(data: data!)!)
                    //loadCounterのデバック
                    print(loadCounter)
                    //loadCounterの出力される値が順番通りにならない
                }
            })
        }

        let transferUtility = AWSS3TransferUtility.defaultS3TransferUtility()

        transferUtility.downloadToURL(nil, bucket: S3BucketName, key: S3DownloadKeyName, expression: expression, completionHander: completionHandler).continueWithBlock { (task) -> AnyObject! in
            if let error = task.error {
                NSLog("Error: %@",error.localizedDescription);
                //  self.statusLabel.text = "Failed"
            }
            if let exception = task.exception {
                NSLog("Exception: %@",exception.description);
                //  self.statusLabel.text = "Failed"
            }
            if let _ = task.result {
                //    self.statusLabel.text = "Starting Download"
                NSLog("Download Starting!")
                // Do something with uploadTask.
            }
            return nil;
        }
    }

}