Of course you'll need an SD Card reader, put the 3DS's SD card into it and load it up on your PC.
Now take any picture on your computer, open it up in paint or any other photo editor and resize it down to 400x240, the 3DS will read anything up to 640x480 but since the top screen is only 240p you won't be able to see any difference higher then that.
Save it to the SD cards image folder which will look something like: DCIM\100NIN03 it varies depending on each system, make sure you save your image as a jpg file, then you have to rename the file with 3 letters, an underscore and 4 numbers, something like HNI_0011.jpg you can use any combo of letters and numbers you like.
That's it, your done, put it back into your 3DS and load up the 3DS Camera software, it may take a little time to refresh the thumbnails depending on how many new images you add and your done. You can even use the graffiti option like I did with this Crysis 2 screenshot below:
Note that images you save this way won't be in 3D for obvious reasons, but when you save a graffiti version of it, the graffiti stickers will appear in 3D. Also there may be a way to save other 3D images to the 3DS using software but I have yet to explore these options, you may also try sharing other peoples 3DS images over the internet, make sure to include the .mpo file the 3DS generates to keep the 3D effect.
Its also fun to note images saved this way will say they where taken on January 1st, 1900 and will show up in there own 1/1 1900 folder.
Well enjoy saving pictures to your 3DS, if you resized your picture as explained above they should only use about 30kb giving you space for millions of images on the default 2GB card.
Edited by Mightylink, Mar 30, 2011, 09:40 PM