Follow the steps below to upload a background image:
On the frontend, in the Admin bar, Customize.
On the backend, click Appearance » Customize.
Navigate to Background Image section.
Click the Select image button to open the WordPress media library.
To upload the background image file, click the Upload Files tab in the upper left corner of the Select image dialog box.
You can drag and drop any image file from your local computer or machine’s file manager, or you can click Upload Files to upload the background image file.
In the File Upload dialog box, navigate to the location of the image you want to use, and then double-click the image to upload it.
Once the file uploaded, the Media Library tab appears with the uploaded file selected.
At the right side of the Medial Library, complete the Title, Caption, Alt Text, and Description text boxes, and then click Choose image button.
Now that you have a Background Image, you will see the available options:
Background Repeat – No Repeat, Tile, Tile Horizontally, or Tile Vertically.
Background Position – Allow image to be positioned Left, Center, or Right.
Background Attachment – Scroll background or display as Fixed.
Click the Publish button to view the uploaded background image on your site.