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

Code Example 1

#example1 {
  background: url(mountain.jpg);
  background-repeat: no-repeat;
  background-size: auto;
}

#example2 {
  background: url(mountain.jpg);
  background-repeat: no-repeat;
  background-size: 300px 100px;
}

Code Example 2

background-size: 100px 50px; /* 100px wide, 50px tall */

Code Example 3

#example1 {
  background: url(mountain.jpg);
  
    background-repeat: no-repeat;
  background-size: auto;

    }

#example2 {
  
    background: url(mountain.jpg);
  background-repeat: 
    no-repeat;
  background-size: 300px 100px;
}

Code Example 4

<head>
<style>
#example1{
  border: 2px solid black;
  padding: 100px;
  background: url(mountain.jpg);
  background-repeat: no-repeat;
  background-size: 100% 100%;
}
</style>
</head>
<body>
<div id="example1">
</div>
</body>

Code Example 5

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

Code Example 6

Background image resize

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