Michiel,

1 - The simplest way to make a flat round roof is to use a large and flat cylinder. (3d models page)
2 - Another way is to add a round room (with ceiling) to the highest level, or add a new level with a round floor.
3 - Maybe you can get inspiration from the Documentation page. There's a PDF on roofs.
4 - This thread explains how you can add files or images.

Hans