Just like the custom header image, custom background image is also a WordPress theme feature. Many WordPress themes come with support for custom background images.
Visit Appearance » Customize to launch the theme customizer. If you can see the Background Image option, then your theme has support for custom backgrounds.
Simply upload an image from your computer or choose one from the media library. Once you have chosen the image you will be able to see the preview and other background image options.
You can tile the image, make it a fixed background, or allow it to be scrolled. Save your changes and preview your website.