You can easily do it yourself if you have computer that runs on Windows as it comes with nice little program called Paint.
Just download any DS cover, open it in paint and plaster the images you provided onto it.
That results in cover in right dimensions and you can do it in about 2-3 minutes.
COME ON YOU CAN DO IT!!!

And no, no cover that is made from Gamefaqs images will make to the sita as the resolution is too low.