Edit in JSFiddle

  img{
    width: 100%;
    height: auto;
  }
<!DOCTYPE html>
<html>
  <head>
    <meta charset="utf-8">
  	<meta http-equiv="X-UA-Compatible" content="IE=edge">
  	<title> Free Software and Gnu Linux Algeria </title>
  	<meta name="viewport" content="width=device-width, initial-scale=1">
    <link rel="stylesheet" href="https://maxcdn.bootstrapcdn.com/bootstrap/3.3.7/css/bootstrap.min.css" >
  </head>
  <body>

		<div class="container">
			<h3 class="text-center">  في البوتستراب 3   <code>.img-thumbnail</code>  مثال على إستخدام صنف الـ    </h3>
      <br>
      <div class="col-sm-6 col-sm-offset-3">
        <img src="https://images.pexels.com/photos/753626/pexels-photo-753626.jpeg?auto=compress&cs=tinysrgb&h=650&w=940" class="img-thumbnail" >
      </div>
		</div>

<script src="https://maxcdn.bootstrapcdn.com/bootstrap/3.3.7/js/bootstrap.min.js"></script>
<script src="https://ajax.googleapis.com/ajax/libs/jquery/3.3.1/jquery.min.js"></script>
  </body>
</html>