Which background image?
Supposing it's the header div background image, place your cursor anywhere in the header div, and in the elements info bar at the bottom of the DreamWeaver screen (above the properties panel) click on div#header to select the div.
In the CSS Styles panel (in the side-bar) double-click on the background-image property (see attached screen-grab).
In the css Rule Definition window that opens up, click 'Browse' to select the image you want to replace the existing one with. You will need to have sized it same as the old one if you are accepting the given header dimensions.
Do the same for any div with a background image you want to change.