Customise the login window
1 Install Deeper
Customising OS X’s Login window is one of the coolest things you can do to personalise your Mac. The default image is your desktop wallpaper with a blur effect, but it’s really just a file called com.apple.desktop. admin.png tucked away in /Library/Caches which you can manually replace, though it’s a lot easier to use Deeper (titanium.free.fr).
2 Change the background
Open Deeper and click the Login icon in its toolbar. Click the Select button next to Background, browse to the file you want to use in place of your desktop background and select it. Click Choose and it will be formatted to the correct dimensions (1920x1080 pixels) and saved in the correct place for OS X to use.
3 Add a custom message
You can also add personalised text to appear above the Login window’s Sleep, Restart and Shut Down buttons. Select ‘Show message in the login window’ option and enter your message in the text box below that. Click Apply. Now choose > Log Out… to view your customised login image and message.