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

Code Example 1

html { 
  background: url(images/bg.jpg) no-repeat center center fixed; 
  -webkit-background-size: cover;
  -moz-background-size: cover;
  -o-background-size: cover;
  background-size: cover;
}

Code Example 2

body {
  background-position: center;
  background-repeat: no-repeat;
  background-size: cover;
}

Code Example 3

background-position: 10px 20px; /*#10px from left 20px from top*/

Code Example 4

body {
  background-position: center;
  background-repeat: no-repeat;
  background-size: contain;
}

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