You might develop a website for your client using WordPress, and want to customize the login page. There are a hacks to modify custom login screen at BinaryMoon, but they also develop a custom login plugin to make your life easier. Just place the plugin into your wp-content/plugins folder, activate, and you already have a custom login screen. You can use different background image for the screen, by uploading it to wp-content/plugins/bm-custom-login/images folder.
Custom login plugin – http://www.binarymoon.co.uk/projects/bm-custom-login/