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

add_action('login_head', 'my_custom_login_logo');

// Outra maneira de inserir Logo na Página de Login
function my_custom_login_logo() {
echo '<style type="text/css">
h1 a { background-image:url(http://mxcursos.com/blog/wp-content/uploads/2012/03/logo1.png) !important; }
</style>';
}

add_action('login_head', 'my_custom_login_logo');