There are a few steps you can follow to add a background image to a single content page in SharePoint 2010:
- Open the page in SharePoint Designer.
- Click on the "Design" tab in the ribbon and select "CSS Styles".
- In the "CSS Styles" task pane, click on "New Style" and select "New CSS Rule".
- In the "New CSS Rule" dialog box, choose "body" as the selector type and click "OK".
- In the "CSS Styles" task pane, click on the newly created style and click "Edit Style".
- In the "Edit Style" dialog box, click on the "Background" tab and then click on "Browse" to select your image.
- Adjust any other properties as needed, such as position, repeat, and size.
- Click "OK" to save the changes.
Your page should now have a background image applied to it.