Just by using some simple CSS you can easily add a custom background image to your website.

All you need to do is grab the URL of your chosen background and insert it into the snippet below.

body {
  background: #fff url('<strong>http://www.yourschool.com/your_bg_image.jpg</strong>') no-repeat top left;
}

Before:

After:

For more information on the CSS Background property try taking a look at this guide