please share your store URL with me. Theme customization detail not available here because it requires coding knowledge.
The link you have sent me is a password protector. So please tell me the password.
Now can you tell me where you want to add a background image? So I can check it from my side if there is any possibility so set background image there or I need to do theme customization.
For your information, theme customization is chargeable.
For a faster resolution, you can contact me through the mail.