If you mean the apple logo that is on the grey screen then Bootxchanger will do that.
If you mean the aurora desktop picture that is behind the login window then go to /System/Library/CoreServices/ and put whatever picture you want in there but it has to be named DefaultDesktop.jpg.
Did you even bother to take the time to read the second post in the thread? If you had you would know that you are wrong. Seriously, it's not like this thread has several hundred posts in it, you were only the fourth poster.