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。

当放到group中的所有请求都完成时，才会回调dispatchgroupnotify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
});