The Finder icon needs to be changed 3 places. First inside
Finder.app in
System/Library/CoreServices/ and
Dock.app the same place. You should also look into the
CoreTypes.bundle in the
CoreServices folder. The Trash icon should be changed there and also in
Dock.app. The reason for this is that the Dock icon would otherwise appear as the standard icon in the Empty Trash dialog and other places, like PathFinder's -Trash on the desktop- ability. Also, if you want to the altered Finder icon to appear in About This Mac, you have to change the Finder icon in CoreTypes.bundle.
ShapeShifter by Unsanity patches these icons live when needed. I've used it because I didn't want to mess around with the CoreServices folder. But really, as long as you replace the files with others that has the same sizes(where PNG's are needed) and names the files correctly(extension/filetype), there will be no problems.
Just remember to repair permissions because I think the system needs these files to have the correct permissions. Even though they are just icons.
In the case of System Preferences you just have to highlight the icon in the Get Info window and press the Delete key. the new icon should appear. You could also change the permissions for System Preferences while you are changing the icon inside the app itself and then set the permissions back. The Dashboard can be changed with CandyBar in the "System" pane. It might be somewhere else too.
Have a look around. If you're brave enough
If you need further instructions, just ask
