You can send files back and forth between a 3ds and a computer by using petit modem, a couple audio cables, and a audio cable splitter. You can look it up online to figure out how to put it together and use these things.
If you're trying to get pictures from something like a camera take its sd card out and copy the picture(s) to the computer and use petit modem. Same works for a 3ds, but the pictures would be found in the DCIM folder.
