To show one background image on all pages of your site:

1. Configuration > User Interface: Dynamic background.
2. Uncheck any existing images.
3. Select “Browse” to upload new image.
4. Background images should be approximately 1200x900px with a 30% jpeg optimization (so they will appear sharp enough but not take too long to load).
5. The background image will automatically resize to always fill either the width or height of the background of your page.