You can do this two way.But I prefer to go for the second way.You need to put this code in your plugins file.
First way:
function my_loginlcustomization() {
echo '<style type="text/css">
h1 a {
background-image: url(' . plugin_dir_url( __FILE__ ).'/login/logo.png) !important;
}
</style>';
}
add_action('login_head', 'my_loginlcustomization');
Second Way(I prefer):
function my_loginlcustomization() {
wp_register_style('custom_loginstyle', plugins_url('/css/login.css', __FILE__));
wp_enqueue_style("custom_loginstyle");
}
add_action('login_head', 'my_loginlcustomization');
Thanks