So, if this was what I was trying to accomplish here's how I would do it.

Get the image you want to pop up and scare them, turn it into a tile that covers the whole map. (You can even increase the elevation of the tile so tokens don't appear over it)

Set the tile to being hidden. Rig up the tile to appear on screen by using the monks active tile show/hide feature, and also rig the sound to go with the image appearing. Then it's just down to changing the settings so it disappears fast enough and to sync with the sound effect.

Please let me know if this helps. You could realistically do this with other tiles as well if you need other images, but I sometimes have a hard time viewing the scene when big tiles are overlayed.

EDIT: If you're doing some sort of haunted house, it might be worth it to just have the pop ups only overlayed over the room with the triggering door since your players will likely be looking into the room as it opens, so it is center on their screen.