AJ Troxell

With the Custom Login Page, you can change the background image of your login page, the background image/colo of your login form div, the main logo image of your login page, and add custom CSS. 
All through a simple interface. Remember to view the Help tab at the top of the settings page for the plugin.

Custom Login Page

Wordpress Custom Login Page allows you to change the background image, logo, form div attributes, and custom css of you Wordpress login page.

prontotools

zkancs

Do some or all of the websites in your multisite network require a custom login logo? Well now you’re in luck!
This plugin allows you to customize the login logo per site, with the option to use the network’s default logo as well.
Once installed, access your theme’s Customize settings to update the logo.
Here is our <a href="https://github.com/prontotools/multisite-login-logos" title="Fork" rel="nofollow ugc">Git repository</a>. Feel free to contribute!

Multisite Login Logos

Easily change the logo on a network site's WP login screen using WordPress's Customize settings.

Custom Login Page vs Multisite Login Logos