ThinkPHP做的，部署在IIS上，上传图片报“上传目录不可写”

public function upload() {        //导入图片上传类        import("ORG.Net.UploadFile");        //实例化上传类        $upload = new UploadFile();        $upload->maxSize = 3145728;        //设置文件上传类型        $upload->allowExts = array('jpg','gif','png','jpeg');        //设置文件上传位置        $upload->savePath = __ROOT__.'/Public/Uploads/Images/';        //设置文件上传名(按照时间)        $upload->saveRule = "time";        if (!$upload->upload()){            $this->error($upload->getErrorMsg());        }else{            //上传成功，获取上传信息            $info = $upload->getUploadFileInfo();        }        $savename = $info[0]['savename'];        $url = $savename;        //保存表单数据，包括上传的图片        $model = M("pkxs");        if (false === $model->create ()) {            $this->error ( $model->getError () );        }        // 更新数据        $list=$model->save ();        if (false !== $list) {            //成功提示            $this->assign ( 'jumpUrl', Cookie::get ( '_currentUrl_' ) );            $this->success ('编辑成功!');        } else {            //错误提示            $this->error ('编辑失败!');        }    }