MPO is a file format made of two (usually) JPEG images to crate one 3D image.
These images can then be viewed on compatibe devices like 3D cameras and the Nintendo 3DS.
Since MPO files are simply two JPEG images, the first JPEG image (usually the left one) can be opened in a normal image viewing or editing program like Windows Photo Viewer or Paint.
To view 3D images using the Internet Browser included with system version 2 for Nintendo 3DS, simply open the image as usual, either by folowing a link or entering the URL manually. If the URL is long, you might consider using an URL shorterner.
This doesn't work with ebmedded images and can therefor only be used tho display one image at a time. If you want to display lots of images you may want to consider using Nintendo 3DS Camera instead (see below).
To view these images in the bult-in Nintendo 3DS Camera application, you will have to place the MPO file in this folder on an SD-card:
# should be replaced with any number, and
X should be replaced with any letter
If you don't have a 3D compatible device which supports MPO files, you can still view the image in 3D or convert it to other formats using an external program.
One such program is StereoPhoto Maker. It can show images interlaced (eg. alternativ rows), anaglyph (eg. red/cyan), side-by-side (including mirrored), flashing. It also support various 3D displays and file formats.