dispatch_queue_t concurrentQueue = dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0);
dispatch_async(concurrentQueue, ^{

    __block NSMutableArray *newImages;

    dispatch_sync(concurrentQueue, ^{
        newImages = [NSMutableArray array];
        // images retrieved using `NSURLConnection sendSynchronousRequest`
    });

    dispatch_sync(dispatch_get_main_queue(), ^{
        // display images to the user
    });
});

q = /* custom serial queue */
urls = /* urls array */;
NSMutableArray *images = [NSMutableArray new];
for (NSURL *url in URLs) {
   NSURLRequest *req = [self imageRequestForURL:url];
   dispatch_async(q, ^{
     UIImage *image = [self imageFromRequest:req];
     [images addObject:newImage];
   }
}
dispatch_async(q, ^{
   dispatch_async(dispatch_get_main_queue(), ^{
     [self applyUpdatesForURLs:urls withImages:images];
   });
}

NSURL *url = [NSURL URLWithString:@"your server address"];
    NSURLRequest *request = [[NSURLRequest alloc] initWithURL:url cachePolicy:NSURLRequestReloadIgnoringLocalCacheData timeoutInterval:5];
    [NSURLConnection sendAsynchronousRequest:request queue:[NSOperationQueue mainQueue] completionHandler:^(NSURLResponse *response, NSData *theData, NSError *error){
        if (error == nil) {
            //do what you want with theData and update the UI
        }
    }];