DownloadTask.m

NSString *imageUrl = @"http://images.apple.com/v/watch/c/images/nav_sport_large.jpg";
    NSURLSessionConfiguration *config = [NSURLSessionConfiguration defaultSessionConfiguration];
    NSURLSession *session = [NSURLSession sessionWithConfiguration:config delegate:self delegateQueue:nil ];

    NSURLSessionTask *downloadTask = [session downloadTaskWithURL:[NSURL URLWithString:imageUrl] completionHandler:^(NSURL *location, NSURLResponse *response, NSError *error) {
        UIImage *downloadedImage = [UIImage imageWithData:[NSData dataWithContentsOfURL:location]];
        dispatch_async(dispatch_get_main_queue(), ^{
            self.imageView.image = downloadedImage;
        });
    }];

    [downloadTask resume];