Customize the Login Screen on OS X Yosemite

If you’re running Yosemite, all you better do is store your desired image as:/Library/Caches/com.apple.desktop.admin.png.

Then, just bail out to apply the change.

If you’re not on Yosemite,things work a little differently.

Customize the OS X Yosemite Login Screen| Jacob Salmela