When you insert the memory card from the 3DS onto a PC, you'll have a folder named DCIM, just like on normal cameras, once you enter that folder you get another folder and there you can add the pictures, I think it needs to be the same format as the other 3DS pics, just look at the extensions (all pics u take in 3D with your 3DS get two copies, a 3D file which can't be seen on the PC and a .jpg or .png file, which format it is, is what I'm not sure atm, just check them when you're adding the pics to make sure they're the same format.