Good question. I just measured a DS manual at 4 1/8" x 4 1/8", so take your picture height (in pixels) and divide it by 4.125 to get the DPI.
There's many official manuals in The Manual Project but combining them with pictures needs to be done manually. Once combined the pdf download button will download the official pdf, while the online viewer shows the pictures. Try to save as .png if you can, they're much larger than .jpg but can be shrunk without losing quality.
One thing though: nintendo provides some manuals on their website ( example, some ds manuals https://en-americas-support.nintendo.com/app/answers/detail/a_id/16905/~/nintendo-ds-manuals ). What i did is i downloaded the pdf, and screenshotted each page (pixel perfectly). Would this count, and if yes, what should i put in "Scan Resolution (dpi)"?