I avoid Bottom margins because they can force a new page even if there is no content.
Also since you mention Images: Think
Box Model
The page is a box with a margin / padding.
Inside is a Body
Inside that is a Div or Paragraph with a picture
If you set the picture to the size of the screen, there are still those outer layers to render.
In the Viewer: Escape to get the Menu: Inspector
You can select a block (tag) and see what the Metrics are