1.
turn off your 3DS
2. on the side of your 3DS there is this little compartment that opens up, open it.
3. remove the SD card, put it in your computer (
make sure that the 3DS is off or your game data could get corrupted!!!)
4. upload the pictures from your SD card
5. put it back in your 3DS once you're done.
you could also do it without removing your SD card by following
this tutorial, but you'll be limited as to where you can upload them