The other option (though LiteIcon is safer) is to change the icon within the Dock.app package. It's in /System/Library/CoreServices/Dock.app/Contents/Resources/ and it's called finder.png. You can do a "Go To Folder" in the Finder 🙂apple:-shift-G) and just paste that path in if you like. Replacing it with another png file will change the icon that appears in the Dock, though be careful to set the permissions back to the original settings.
jW