Changing Apple applications icons

I installed Snow Leopard today on a Mac Mini. Installing without worries.
By cons, many of my preferences (particularly in terms of appearance) were changed. As the icons of Apple applications.
Only problem is that since I installed Snow Leopard, I can not change their icons as I did before.
My method was simple: Opening a png with Preview / copy, open information application / selection of the icon and paste.
But, I can not copy directly with Preview (the feature is grayed out, or it should select the image with the right tool), and I can not paste on ALL Apple applications.
So I find myself with a dock or only the applications Apple is impossible to change.
Someone an idea?

To change the icons of the Apple applications (Safari, Mail, iTunes, etc ...):

1. Find the original icon (right click on the text / display the package contents / Contents / Resources)
2. The icon has a. Icns (*)
3. Copy and paste the icon into a backup folder to be able to recover in case.
CAUTION:
The icon should replace the original should be mandatory in. Icns (**), and have the same name (be careful with uppercase and lowercase)
4. Rename your new icon.
5. Click on the icon to select it, then cmd + c (copy)
6. Cmd + v (paste) into the Resources folder of the application (authenticate if requested)

(*) Names of some icons of origin:
For iTunes: iTunes.icns
For Safari: compass.icns
Mail To: app.icns
QuickTime X QuickTimePlayerX.icns