The secret to seeing these pictures is to focus your
eyes straight ahead, don't look at one point, try to
get each eye looking at a different place, it is easy
with a paper picture, you hold up to your face till
it touches the nose, keep looking at the same bit
as you slowly move the image away, you will see
hidden object much quicker.
With practice, you can look at one and unfocus your
sight almost immediately.
Mike.