I tried adding a background image to a slide and one of two things happen:

- the image is too big, spilling out over the edge of the slide.
-the image is too small, not filling the whole slide.

when i try to resize there is always either some spill over the edge, as when the width is correct the height is too much, or some slide visible, for the same reason.

How do i add an image as background correctly, so it is the correct slide size and will scale with the slide depending on monitor resolution?