How to reply in a funny way to an apparent photobomb from my crush?

What is your end game- to take your crush to the next level I would guess. So my response would be more open ended take it more like a challenge. An eye for an eye kind of thing. You could say something like- " You fell victim to one of the classic blunders- The most famous is never get involved in a land war in Asia- but only slightly less well known is this: Never go against ( you name here) when photo bombing is on the line! HAHAHAHAHAHAHAHAHA Prepare to for my revenge! "

I hope you get the reference to the movie Princes Bride.( if you don't that is very sad and you must sit down and watch the move- but hopefully you see where I am going with this--- with an opening like that you can no have some fun with her in person too. Such as the next time you see here do the "I am watching you" gesture or as you walk by say something like " never mind me, I am just going to lurk back here and wait for my opportunity to strike." You can turn this into a fun inside joke between the two of you.

Hmm well first was it a silly photobomb on purpose or was she just accidentally in the picture? What's happening in the picture?