I remember reading a study on this and they found most people imagine the characters to be remarkably similar. It really isn't that surprising to me either. Books can be quite descriptive of a characters physical features. So as long as the writing is detailed enough, readers should have some what similar image in their head of what characters look like. I'm currently unable to find the study again, but will update if I do.

Now as for the scenery you are probably correct. It's a lot harder to describe every detail of a scene.