I try to set the lock screen wallpaper from my application but i have a problem.

I use below code

        //set the lock screen background image
       String filepath = App::GetInstance()->GetAppResourcePath() + L”screen-density-xhigh/back.jpg”;
        String key(L”http://tizen.org/setting/screen.wallpaper.lock”);
        SettingInfo::SetValue(key,filepath);

but the result is not the expected. The lock screen wallpaper changes but with a black one and not the  specified wallpaper.

I have the privilege  http://tizen.org/privilege/setting in the config.xml