BisonTech

This plugin replace WP default admin/login/register page. 
The technique used is pretty brutal but the best currently possible, the plugin replaces the modified files automatically. 
To make the plugin run the web server must be able to write to the wp-admin directory. 
more info @ http://www.bisontech.net

LogoReplacer

This plugin replace WP default admin/login/register page.

sciweavers

A TinyMCE plugin that allows you to insert free clipart images to posts on your WordPress site. During writing a post, if you click the i2clipart plugin button, a popup dialog will show up containing a search box and a color palette. You can search for an image by typing the proper keywords or provide its dominant color through the color palette. The plugin will search i2clipart.com and display paginated search results in the form of small thumbnails. When you click a thumbnail, the associated image will be inserted into your TinyMCE editor. Every image comes in four different sizes (64px, 128px, 256px, 512px). Five buttons are displayed under every thumbnail. The first four buttons are labeled (S, M, L, X) describing image size in PNG format. The last button will show a set of thumbnail images having several color variations of the selected thumbnail. All provided images are 100% royalty free public domain clipart images.

i2Clipart

TinyMCE plugin to search and insert royalty free public domain images from i2clipart.com to your page or post.

LogoReplacer vs i2Clipart