Try it Yourself - CSS box-shadow6

<!DOCTYPE html>
<html>
<head>
<style>
#boxshadow {
   position: relative;
   -moz-box-shadow: 1px 2px 4px rgba(0, 0, 0,0.5);
   -webkit-box-shadow: 1px 2px 4px rgba(0, 0, 0, .5);
   box-shadow: 1px 2px 4px rgba(0, 0, 0, .5);
   padding: 10px;
   background: white;
}

/* Make the image fit the box */
#boxshadow img {
   width: 100%;
   border: 1px solid #8a4419;
   border-style: inset;
}

#boxshadow::after {
   content: '';
   position: absolute;
   z-index: -1; /* hide shadow behind image */
   -webkit-box-shadow: 0 15px 20px rgba(0, 0, 0, 0.3);
   -moz-box-shadow: 0 15px 20px rgba(0, 0, 0, 0.3);
   box-shadow: 0 15px 20px rgba(0, 0, 0, 0.3);
   width: 70%;
   left: 15%; /* one half of the remaining 30% */
   height: 100px;
   bottom: 0;
}
</style>
</head>
<body>

<div id="boxshadow">
  <img src="rock600x400.jpg" alt="Norway" width="600" height="400">
</div>

</body>
</html>