In CSS, the background-image property sets a background image of your choice for a given selector, as seen below.

.hero {
  background-image: url("https://s3.amazonaws.com/codecademy-content/projects/make-a-website/lesson-2/bg.jpg");
}

The CSS rule above assigns the image hosted at

https://s3.amazonaws.com/codecademy-content/projects/make-a-website/lesson-2/bg.jpg

to elements that have a class attribute set to hero.

To control the size of the chosen background image, use the property background-size as seen below:

.hero {
  background-image: url("https://s3.amazonaws.com/codecademy-content/projects/make-a-website/lesson-2/bg.jpg");
  background-size: cover;
}

Here, we have specified that we want the image to completely cover elements with the .hero class.