You can add a background image via the WordPress customizer however it displays on every page of your site. What if you want a different image on a specific page?
You could use code to remove the image and then add a different one back. Thats one solution which works fine however there is a better solution.
Here’s another which is one of the best and one which many of the best background plugins use.
Add to the end of your child themes functions file.
Then add your images(s) to your child themes images folder making sure the file name and extension match whats in the code.
You’ll also need to make sure the conditional tag targets the page/post or archive you want to change.