plotting orbits from photos?

I see articles where they have taken 2 pics of an object,(eg a comet or
asteroid) and the movement from pic 1 to pic 2 is only a part of an inch as
measured on the photos. How do they calculate the orbit of an object with
so little information? I'm not looking for too much math, mostly just a
high level explanation maybe a drawing or two. Seems like you'd have to
have a lot more info before you could say where the object was and where it
was going.
Eric