Background Image on Webpage

I am working to develop a small web app with netbeans 8.2. The background image is not shown on the web page by using css property. But when i copy and paste the same code in head elements the image shows.

'''

<style>
            body {
                background-image: url('assets/images/back.png');
                background-repeat: no-repeat;
                background-attachment: fixed;
                background-size: cover;
            }
        </style>

''' and the css file have '''

body{
    background-image: url('assets/images/back.png');
    background-repeat: no-repeat;
    background-attachment: fixed;
    background-size: cover;
} '''

while i have a css file link in my head element '''

<link href="assets/css/bootstrap.min.css" rel="stylesheet">
    <!-- Optional Bootstrap Theme -->
    <link href="assets/css/bootstrap-theme.min.css" rel="stylesheet">
    <link href="assets/css/Mycss1.css" rel="stylesheet" type="text/css"/>//this is css file having body code'''

Regards