Ok, thank you for clearing that up. I would advise against that because layering images over images hurt the eyes, and it will look weird when scrolling down.
But if you insist on doing this, go to your dashboard -> settings -> theme and scroll down to the "custom css" section and paste this (change the image url in the code)
Code:
body {
background-image: url("https://yourbackgroundimageurl.com");
}