The Brief: A photobomb is a comical, unintended presence in a photo.

DEEP DIVE
To photobomb someone is to place oneself in the background or foreground of a photo, often as a last-second joke.
The ultimate photo bomb. pic.twitter.com/yspq11vjmk
— Lindsay Guentzel (@LindsayGuentzel) January 29, 2019
Best Photobomb Ever!. pic.twitter.com/JGJnWxDIFN
— Land of cuteness (@landpsychology) February 6, 2019