Either you’re tired of your old WordPress login page, you want something new for your WordPress blog, or hack your logo into it, here is a way you can customize your login screen. It uses simple CSS code that you don’t need to meddle with CSS core files of your blog. You can not only set the logo in the login page, you can also replace the background image. When editing to create a customized login CSS, you need to take note that you should put !important property so that it won’t get overridden by WordPress.
Get instructions here: http://www.cheesetoast.co.uk/customize-wordpress-login-screen-logo-css/