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