Sometimes you just want to change the logo on the WordPress login screen. There are plugins for this, but they usually do a number of other things you may not want. If that’s the case, you can use this.

function custom_login_logo() {
    echo '<style type="text/css">
        h1 a { background-image:url('.get_bloginfo('template_directory').'/images/my-custom-login-logo.png) !important; }
    </style>';
}
add_action('login_head', 'custom_login_logo');