High res screenshot for app submission

[Srikanth3d]

I have created a app with Windows Phone App Studio and tried to submit it to Store
But the problem is I have to provide screenshot of my app at resolution of 1280x768 , I have only Lumia 520 in which screenshot is 800x400
Without providing screenshot i cannot go forward.
I do not have any knowledge about Visual studio etc
I have tried screenshot with Nokia RDA but it too takes low res screenshot.
Is there a way out of this problem?

 

[Expression2]

Port it to Visual Studio and run the app under the emulator. Then take a screenshot in the emu.