Option to set a background image for the site is a premium feature available with Astra Pro Addon plugin. To use these Pro features, you need to have the Astra theme along with the Astra Pro Addon installed on your website.
- Activate the Site Layout and Colors & Background module available with Astra Pro Addon plugin.
- Set site layout to Padded or Max Width.
- Navigate to Appearance > Customize > Global > Colors > Base Colors
- You will find an option to set the background image to the site.
- Improvement (*) from Astra theme and Astra Pro v2.4.0 you will find a responsive option besides the Background option.