It is possible to place your own photographs onto the 3DS camera, even 3D photos. They must be .jpg (2D) or .mpo (3D), and preferably of a 400*240 (widescreen) or 320*240 (fullscreen) resolution.
This is how you do it:
1) Name the image anything you want, as long as you follow these rules: 8 letter file name, first four letters must be Capital letters, underscores or numbers, last four letters must be numbers. (ex.: ES1_1001.mpo)
2) Place the pictures into a folder named with 8 Capital letters/underscores/numbers. (ex.: /PICS_ES1/)
3) Place the folder into the DCIM folder on your SD card.
You now have the picture viewable, and uploadable, from your 3DS. Funnily enough, this is the same reason why you can download pictures from this, or any, site, as long as the file follows the rules I just listed.
