How a background image can be added?

The background-image CSS property sets one or more background images on an element.

The background images are drawn on stacking context layers on top of each other. The first layer specified is drawn as if it is closest to the user.

The borders of the element are then drawn on top of them, and the background-color is drawn beneath them. How the images are drawn relative to the box and its borders is defined by the background-clip and background-origin CSS properties.

div {
  background-image:
      url("https://mdn.mozillademos.org/files/6457/mdn_logo_only_color.png");
}

.catsandstars {
  background-image:
      url("https://mdn.mozillademos.org/files/11991/startransparent.gif"),
      url("https://mdn.mozillademos.org/files/7693/catfront.png");
}

Source: https://developer.mozilla.org/en-US/docs/Web/CSS/background-image