Open up the site. Select the home page.
- Select the home page from the navigation bar.
- Select the inspector tool.
- Select the document inspector.
- Click on appearance
In the baner area, select Image Fill. You need the right sized image to do this well.
Navigate to an image, click insert
And your image replaces the standard one. You can also lose the title, tagline, and footer with the layout checkboxes above.