Sneak into the shot

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

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.