Customize the OS X Yosemite Login Screen
The login screen on Mac is pretty boring, but if you want to customize it, blogger Jacob Salmela will show you an easy way to do it.
If you are using Yosemite, all you have to do is save the desired image as: /Library/Caches/com.apple.desktop.admin.png . Then just log out to apply the change. Salmela notes that the best wallpapers tend to be aligned to the right or left, so login fields don’t close, and brighter images work best because the login window darkens them. Salmela includes a Photoshop template to help you create a solid image, as well as some cool examples to get you started. If you’re not in Yosemite, things work a little differently .
Customize the OS X Yosemite Login Screen | Jacob Salmela