Odoo 9 : Adding background image for login screen

1) create a new xml file as 'webclient_templates.xml' an add below code.
<?xml version="1.0" encoding="utf-8"?>
<openerp>
<data>
 
        <template id="login_layout_inherited_your_module" inherit_id="web.login_layout">
            <xpath expr="." position="inside">
                <link rel="stylesheet" href="/your_module/static/src/css/your_css_file.css"/>
            </xpath>
        </template>

        </data>
</openerp>

2) Create a new .css file your  module and add below code.

.container {
background-image: url('/your_module/static/src/img/your_image.jpg');
}