Pop up in someone's shot

• Drop-in shot?
• Laugh-inducing pic
• Picture with a surprise ending?
• Spoil by making faces, as in a stranger's camera shot
• Sneak into the shot
• Block a shot?
• Ruin, as someone's selfie

Wiktionary Word definitions in Wiktionary
n. 1 An act of photobombing. 2 A photo containing someone or something that is photobombing. vb. (context transitive English) To unexpectedly appear in a photograph, especially so as to ruin the picture.