To change the icons that appear in the Dock for Trash and Finder, you need to edit a couple PNG images inside the Dock application.
Browser to the following location.
/System/Library/CoreServices/Dock.app/Contents/Resources
To get into Dock.app, you are going to need to browse to /System/Library/CoreServices/ first, and then Control+click (or right-click) on the Dock.app application. When you get the contextual menu, choose "Show Package Contents". The contents of the Dock app will open in a new Finder window.
Once there, open the Contents folder, and then open Resources. The files are in here. You are looking for:
trashempty.png
trashfull.png
Also, you'll find the icons for Dashboard and the Finder:
finder.png
dashboard.png
These files are owned by the system, so, if you try to delete them, you are going to be prompted for the username and password of an administrator.
Make backups of these files before you edit them! In fact, making a copy of the Dock.app application, located in the CoreServices folder, is a good idea. That way, you can just copy it back and replace everything that you change. Then, replace the files for the icons with new files of the same names.
When you are done, restart the Dock. Open Terminal, and type the following line, followed by the Return key.
killall Dock
You may need to move something to the Trash, or empty it, to get the icon changed.
I just found this method somewhere else, and re-wrote it to be a little easier to understand, and so that it uses the Finder to do all the work. If anyone else has any corrections to what I've written above, PLEASE post them. Thanks.
EDIT: Meant to post the article where I found this tip...
Here it is:
http://interfacelift.com/icons-mac/articles/dock_10-3-x.php