Some time ago I was bidding a project for a plugin to allow the customization of the login page, I didn’t win the project but I wanted to build the plugin so I did some research and started to code it.
This plugin allows you to define custom header and footer templates to be used in the login/logout, lost password and register pages to match the design of your site. In the custom templates you can include CSS styles to set a background image, the size and colors of the login form, etc…
How to use this plugin
After activation you will have a new settings page for “Custom Login pages”, in this page you add filenames of the header and footer templates to be used whenever a login screen is displayed.

Custom login pages option page
The templates have to be in the current theme directory, you can name the templates whatever you want and if the plugin does not find the templates it will use the default header.php and footer.php.

Custom login page

Custom register page
The plugin is compatible with Cimy user extra fields. You can see more screenshots of this plugin and the download link in the Custom login pages page