The best photobomb is the one nobody spots until the second watch. Upload an ordinary selfie — the more ordinary the better, a hallway behind you, a lamp on, nothing staged — and it comes back as a five-second clip where something pale drifts in from the back of the room, crosses behind you and is gone. You stay exactly as you were, right up until your eyes flick sideways at the last moment.
It's deliberately kept on the funny side of scary. No blood, no injuries, no faces coming at the camera. That makes it something you can put on any feed in October without a content warning.