Check out example codes for "background css". It will help you in understanding the concepts better.

Code Example 1

body {
  background: #ffffff url("img_tree.png") no-repeat fixed right top;
}

Code Example 2

/* Color (Example: red): */
html,body {
  background-color: red;
}

/* Image (Example: your.picture): */
html,body {
  background-image: url("your.picture");
}

Code Example 3

/* Answer to: "adding background image css" */

/*
  The background-image property sets one or more background images
  for an element.

  By default, a background-image is placed at the top-left corner
  of an element, and repeated both vertically and horizontally.

  Tip: The background of an element is the total size of the
  element, including padding and border (but not the margin).

  Tip: Always set a background-color to be used if the image is
  unavailable.
*/

/* Set a background-image for the <body> element: */

body {
 background-image: url("paper.gif");
 background-color: #cccccc;
}

/* Set two background images for the <body> element: */

body {
  background-image: url("img_tree.gif"), url("paper.gif");
  background-color: #cccccc;
}

Code Example 4

/*CSS background Property

Example
Set different background properties in one declaration:
*/
body
 { 
   background: lightblue url("img_tree.gif") no-repeat fixed center; 
 }

Learn ReactJs, React Native from akashmittal.com