Do you want to change the login wallpaper on your Mac OS X? It is as simple as replacing an image in the file system.
First, open the File Explorer.
Select your hard drive from the Devices tab.
Click on the System folder.
Click on the Library folder
Find the CoreServices folder and click on it.
In that folder you will find a DefaultDesktop.jpg file. Replace the old file with the new file that you want to use as your new Mac OS X login screen.
Before moving the new file into the CoreServices folder, make sure you name it DefaultDesktop.jpg!