To go deeper!
body {
background:url('your_image_url');
}
Will give:
If the image you use is small.
It will be better if you use some others CSS background properties:
background-position (more info >)
background-repeat (more info >)
background-position (more info >)
background-attachment (more info >)
and many more!
Here an example of a full size image background, centered and fixed:
https://codepen.io/mangasource/pen/MzQyLr
Or you can use a little image (textures or subtle patterns) and repeat it, like this:
https://codepen.io/mangasource/pen/mQXPNJ
You can do many things with CSS!
Have a good day!