gerund or present participle: photobombing
  1. spoil a photograph of (a person or thing) by unexpectedly appearing in the camera's field of view as the picture is taken, typically as a prank or practical joke.

(verb)- to drop in a photo hop in a picture right before it is taken.
Above are two definitions I found online.  Who doesn't love a great photobomb pic.  Before the internet and smart phones it was rare to see any ones photobomb pictures unless it was in the news or you are a celebrity.  People have been doing it since cameras were first invented, but no one really new about them or enjoy them until they could send them out hundreds and thousands of people via email, Facebook and other social media sites.  My first camera could take 12 pictures and if you used the flash, the bulb would have to be replaced.  This was about 1970.  The film had to taken to a special photo shop and you would get your pictures in about 7-10 days.  The pictures were about a $2 each and only your family and a few friends ever got to see them.  So if you jumped in a shot or made a funny face, your family laughed or told you that you ruined the picture.  Today, you can take hundreds even thousands of digital photos for "free" on your digital camera, smart phone in a few days and share them to hundreds, thousands, and more.. of people in just a few seconds around the world.  I look forward to the next 10-20 years to newer digital pics, 3d pics, who knows what's next!  

One of my favorites: