WordPress.org

This plugin enables you to change easily the backend login logo.

In addition you can define different logos for each domain, if you're using more than one with your WordPress installation.

Without changes the image "default.jpg" from the folder "/wp-content/plugins/backend-logo/logo/" will be used.

Your own domain-logo you can define by creating a JPG file. E.g. for the domain "test.com": /wp-content/plugins/backend-logo/logo/test.com.jpg or /wp-content/plugins/backend-logo/logo/test.com.gif

Find out more at: http://www.mretzlaff.com/wordpress-plugins/wp-plugin-backend-logo-wordpress-login-logo/