A milk bomb happens usually during gay sex, it results in the top cumming a massive amount (more liquidy than the normal cum) and that's why it's known as a milk bomb.
John: did you see that guys Milk bomb on twitter?
Frank: dude it was fake how do you now know!
Jumping into an unsuspecting person photo, referring to like how a bomb would drop on unsuspecting people, hence the name.
Photographer: 3...2...1...chee-
Photo bomber: *jumps in frame*
Photographer: hey man! you just photo bombed our picture!
Photo bomber: and I'll do it again