You can setup the background image for the entire website from the Customizer > General Styling section.
Change the Site Background Image
You can click on the “Select Image” button. It will pop open a window, where you can choose to upload a new image from your computer. Or choose from any of the previously uploaded or used images, via Media Library tab.
As you upload and select the image to use for your site background, you will see your website will have this image as your new site background on the other side of the screen.