this one. www.goreydetails.net/ima...

But those aren't scary...

yeah they change when you walk by, they don't morph when you just stand there though you have to be moving

those pics just show you what it's going to look like before and after

It doesn't have to make them crap themselves. An uneasy double-take will suffice.

Those usually just get a "oh the picture changed" double take. That's why a lot of them make really loud sounds to startle people.