I haven't seen a Nintendo DS Lite psd template - might as well be the first, eh?
Please, please, please - if you use this, send some love by way of credit in your deviation.
Make your kick-ass Nintendo DS wallpaper the appropriate size and compress it into a .zip or 7z (or some other compressed type).
Open the psp psd template.
drag your wall into the labeled layer(s) (see below), nudge it into position and save as a .jpg. Note: due to the 2 screens you can either make 2 256x192 walls or 1 256x476 (there are 92 lines separating the 2 screens) wallpaper.
Submit your zip file (make sure there are 2 wallpapers for top and bottom) and use this template-ized version as the preview.