A jump scare can be audio or video. A combination of both is usually more effective, but it's like when somebody sneaks up behind you while you're focused and yells "BOO". You're going to jump in the same way you would jump from an engineered jump scare in a movie, video game, or online thing. When thinking about movies though, what really makes you jump? Generally a loud noise shattering an otherwise silent moment. It isn't necessarily scary, but it's incredibly surprising even when expecting it. As well, even without the video, the loud noise out of nowhere would likely make you jump.
When it comes to video, certain things can certainly add to the jump. Usually a surprisingly morbid image, or something that looks close enough to human to make you believe that this thing could actually exist despite being disfigured or unnatural, will be used, combined with a loud scream. The scream likely gets you first, but the image flashing up also does a number as that is just as surprising, breaking visual focus, or what could be equated to almost a visual silence.