body { 
  background-image: url("https://wallpapercave.com/wp/wp2360203.jpg");
  background-repeat: no-repeat;   
  background-attachment: fixed; <!-- this is to make sure even when you scroll the image remains static over the background -->
}

This is my first answer on this website and I'm a beginner and a fresher in college, so feel free to criticise any mistakes in my answer