Copy the following piece of code and past it on your functions.php file:
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');
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