Photobomb (or Photocrash): v., adj.; the act (to photocrash, to photobomb) of anonymously appearing in the background of a planned photograph. Participants often pop up in the foreground or background just as the photograph is taken. Participants are often strangers, and are sometimes not human.Photobombing is not a new phenomenon, but has seen a resurgence in popularity with the aid of various blogs and celebrities. Actor Michael Cera (Juno, Arrested Development) loves to pop into the background of photographs, and a curious, brazen squirrel popped into the foreground of two tourists last year.
Here is Cera discussing his hobby:
A reverse photobomb is when a person poses near an unsuspecting person for amusement and documentation. (Some images on that link are NSFW.)
Photobomb, Urban Dictionary definition
This is Photobomb
Cheeky Squirrel Photo Crasher, Intelligent Travel
Top 23 Reverse Photo Bombs on the Net, Comedy Central/Tosh.0