GCD

    dispatch_queue_t concurrentQueue = dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0);
    dispatch_async(concurrentQueue, ^{
        __block UIImage *image = nil;
        dispatch_sync(concurrentQueue, ^{ /* Download the image here */
            /* iPad's image from Apple's website. Wrap it into two
             lines as the URL is too long to fit into one line */
            NSString *urlAsString =
            @"http://a.hiphotos.baidu.com/image/w%3D2048/sign=e9e1e60e5b82b2b7a79f3ec40595caef/b58f8c5494eef01f728c66cfe2fe9925bc317d1b.jpg";
            NSURL *url = [NSURL URLWithString:urlAsString];
            NSURLRequest *urlRequest = [NSURLRequest requestWithURL:url];
            NSError *downloadError = nil;
            NSData *imageData = [NSURLConnection
                                 sendSynchronousRequest:urlRequest
                                 returningResponse:nil
                                 error:&downloadError];
            if (downloadError == nil && imageData != nil){
                image = [UIImage imageWithData:imageData];}
            else if (downloadError != nil){
                NSLog(@"Error happened = %@", downloadError); } else {
                    NSLog(@"No data could get downloaded from the URL.");
                }
        });
        dispatch_sync(dispatch_get_main_queue(), ^{
            /* Show the image to the user here on the main queue*/
            if (image != nil){
                /* Create the image view here */
                UIImageView *imageView = [[UIImageView alloc]
                                          initWithFrame:self.view.bounds];
                [imageView setImage:image];
                /* Set the image */
                /* Make sure the image is not scaled incorrectly */
                [imageView setContentMode:UIViewContentModeScaleAspectFit]; /* Add the image to this view controller's view */
                [self.view addSubview:imageView];
            } else {
                NSLog(@"Image isn't downloaded. Nothing to display.");
            } });
    });