Mark Jaquith

This plugin allows you to customize the logo on the WordPress login screen. There is zero configuration. You just drop the logo file into your WordPress content directory, named <code>login-logo.png</code> and this plugin takes over.
Note that you should use a transparent background on the PNG image, crop it tightly (no padding pixels) and use a width of exactly 312 pixels for best results. Wider images will be downscaled in modern browsers, but it isn’t recommended to rely on that.
This plugin also works in the <code>mu-plugins</code> directory.

Login Logo

Customize the logo on the WP login screen by simply dropping a file named login-logo.png into your WP content directory. CSS is automatic!

Francesco Campus

Tired of manually customizing every wp-login.php page on every website?
Say no more.
Just install and activate FC Login Customizer and let the plugin automagically handle some nice customizations for you!
Core features:
<ul>
<li>Automatically display your brand logo on the website’s login page</li>
<li>Automatically link the logo to the website’s homepage</li>
<li>Automatically change the hidden H1 title to the website’s name</li>
</ul>
Most people do not like that the login screen (wp-login.php) contains the WordPress logo and a direct link to wordpress.org. 
Clients keep asking to change the logo, SEOs keep asking to change the hidden H1 title, UX Designers keep asking to change the link that wraps the login logo… 
With this plugin, you can do all this stuff at once by just installing it!
<h3>What’s next</h3>
Next update may include the additional features listed below:
<ul>
<li>Official support for .svg logos</li>
<li>HTML markup preview before settings save</li>
<li>Fine tuning on logo width/height</li>
<li>Other features based on community requests</li>
</ul>

FC Login Customizer

Automatically customize the login screen with your brand logo, the associated link when clicked and the hidden H1 title inside the page.

Login Logo vs FC Login Customizer