Hadouken is one of the latest memes-slash-photography crazes. You can see a sample above. The word is a Japenese neologism that roughly translates to "surge fist." The pose is from the game Street Fighter, a popular title in the 1980s.
In Japan, hadouken is called "makankosappo." (In translation it means "demonic piercing light murder gun.") Though the makankosappo is derived from a pose from Dragon Ball-Z. In the US, both moves, and other videogame derived stunts, are called hadouken.
Since I feel like I'm not being clear (and I love that this was started by girls!) I'll let Mashable explain, too:
The meme's spark was kindled in Japan when schoolgirls started posting pictures of themselves throwing balls of lightning at each other, Street Fighter-style. And that's all there is to it. Imgur user grimlockt then kindly pointed it out to the rest of us so we could all beneofit from a new photo trend.Seems safer than planking!
Hadouken, Flickr blog
Hadouken Meme Strikes Internet Like Lighting, Mashable