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!

diablue

Plugin allows admin to upload custom logo for WordPress login page.
After activating the plugin, follow these steps:
<ol>
<li>Go to Settings -> Custom Login Logo</li>
<li>Click “Choose Image”</li>
<li>Select or upload your logo</li>
<li>Click Save Changes</li>
<li>Refresh the login page to see the new logo</li>
</ol>

Diablue Login Customizer

Easily add a custom logo to your WordPress login page using the built-in media uploader. Change WordPress login logo from admin panel.

Login Logo vs Diablue Login Customizer