Then open Terminal and type the following to backup the original file:
sudo cp /System/Library/PrivateFrameworks/LoginUIKit.framework/Versions/A/Frameworks/LoginUICore.framework/Versions/A/Resources/appleLinen.png ~/Desktop/
And then this to copy over the Apple logo with your new logo:
sudo cp /Users/YOUR_USER_NAME/Downloads/BBRocks_256x256.png /System/Library/PrivateFrameworks/LoginUIKit.framework/Versions/A/Frameworks/LoginUICore.framework/Versions/A/Resources/appleLinen.png
Since you are running sudo, you will have to enter your password. This is due to the fact you are copying over the original file which is owned by root.
Now your login window should look like something like this: