Changing backgrounds on the Login/Register pages
Go to your Website's Admin Panel -> Themes -> Theme Editor.
Open the current theme's Editor (if you don't have a custom theme created, create one with your base theme). Once in the Theme Editor, go to the "Website Background" area. Ensure that your background image is uploaded to the first slot. If it's uploaded to the second or third slot, it will break the image on some of your system pages.