Photo Ninja 

A photo ninja is a person who, rather sneakily, poses in the background of a photo, unknown to the subjects of said photo. Photo ninjas can be anybody, though they typically are the kind who dance with themselves at parties, talk to the corner, engage in no social action, etc. They are nothing like real ninjas, except that they are silent and they deal a fatal blow; that being the ruining of photos everywhere.
Today, the modern photo ninjas is of the Facebook variant, which means that their main goal is to boost their Facebook "pictures of" count. They lie awake at night, sweating, maybe panting, in anticipation of an email telling them that "______ has tagged a photo of you on Facebook". At which point the pump their meaty fists in the air and celebrate this victory. Thus, they focus their beady eyes on the soon to be marked comment space. Pounding the F5 key in hopes of a comment from a hot girl who's photo he ruined.
Such is the eventful life of a Photo Ninja

Girl1 (commenting): OMG we look so fiiiiine!!!1!! I love you!
Girl2 (" "): I knooowww! Hold up, is that richard on the backround?? OMG he is so nasty =O
Richard (" "): Hey girlllzz, like me and my fine threads with you fine-looking ladies. Hot damn we look fine
Girl1 (" "): I think I just threw up

A common Facebook Photo Ninja springs into action...and is shot down

Photo Ninja by SpicyyMchaggis May 12, 2008

Photo Ninja 

A person who tries to blend into the background and takes lots of pictures of a group of people/ one person. This person usually takes pictures of celebrities or cosplayers.

cosplayer 1: We are being photo ninja'ed again...

cosplayer 2: I wish they would just ask us to pose for a picture cause we will

cosplayer 3: Should we smile at them or flip them off...

Photo Ninja by hetaliaftw! July 25, 2010

Photo Taking Ninja

A person who takes many photos at awkward moments whilst going unnoticed. The people in the photos have no idea what they were doing in the photos captured

Gemima Louisa is a photo taking ninja. Evidence visible from the China album on facebook.

Photo Taking Ninja by R.M Murphy December 13, 2012

Photoninja

A synonym for a Photobomber.
One might refer to the Photoninja as a more well refined version of your typical Photobomber.
Sometimes you don't notice the Photoninja until it's way too late...and if you look closely at the picture, there might be more them very well hidden.
Plus Ninjas are cool - numerous websites have verified that!

Biff: Hey, how that person get into my picture? I've never noticed that before!
Kipper: It's a "Photoninja". No one ever sees them come and go until it's too late
Biff: Wow, that's like photoshopping someone into the pic, but for real. Sweeeeet!

Photoninja by Albert H October 16, 2008