GCD：
使用Dispatch Group追加block到Global Group Queue,这些block如果全部执行完毕，就会执行Main Dispatch Queue中的结束处理的block。
dispatch_queue_t queue = dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0);
dispatch_group_t group = dispatch_group_create();
dispatch_group_async(group, queue, ^{ /*加载图片1 / });
dispatch_group_async(group, queue, ^{ /加载图片2 / });
dispatch_group_async(group, queue, ^{ /加载图片3 */ });
dispatch_group_notify(group, dispatch_get_main_queue(), ^{
// 合并图片
});

需要注意的：dispatch_group_wait是同步的所以不能放在主线程执行。
补充： dispatch_group会等和它关联的所有的dispatch_queue_t上的任务都执行完毕才会发出同步信号(dispathc_group_notify的代码块block会被执行,group_wait会结束等待)。也就是说一个group可以关联多个任务队列；下面给出示例：

• (void)groupSync2
  {
  dispatch_queue_t dispatchQueue = dispatch_queue_create("ted.queue.next1", DISPATCH_QUEUE_CONCURRENT);
  dispatch_queue_t globalQueue = dispatch_get_global_queue(0, 0);
  dispatch_group_t dispatchGroup = dispatch_group_create();
  dispatch_group_async(dispatchGroup, dispatchQueue, ^(){
  sleep(5);
  NSLog(@"任务一完成");
  });
  dispatch_group_async(dispatchGroup, dispatchQueue, ^(){
  sleep(6);
  NSLog(@"任务二完成");
  });
  dispatch_group_async(dispatchGroup, globalQueue, ^{
  sleep(10);
  NSLog(@"任务三完成");
  });
  dispatch_group_notify(dispatchGroup, dispatch_get_main_queue(), ^(){
  NSLog(@"notify：任务都完成了");
  });
  }
  NSOperation：
  // 创建队列
  NSOperationQueue *queue1 = [[NSOperationQueue alloc] init];
  // 添加操作
  NSBlockOperation *op1 = [NSBlockOperation blockOperationWithBlock:^{
  NSLog(@"download1----%@", [NSThread currentThread]);
  }];
  NSBlockOperation *op2 = [NSBlockOperation blockOperationWithBlock:^{
  NSLog(@"download2----%@", [NSThread currentThread]);
  }];
  NSBlockOperation *op3 = [NSBlockOperation blockOperationWithBlock:^{
  NSLog(@"download3----%@", [NSThread currentThread]);
  }];
  // 设置依赖(保证op3在op1和op2都执行完之后再执行)
  [op3 addDependency:op1];
  [op3 addDependency:op2];
  [op1 start];
  [op2 start];
  [op3 start];