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