Mac: The Mac login screen is pretty boring. If you want to customise it, blogger Jacob Salmela shows off a simple way to do it.
If you're running Yosemite, all you need to do is save your 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 justified to the right or left so the login fields aren't covered up, and brighter images work best because the login window dims them. Salmela includes a Photoshop template to help you make a solid image, as well as a few geeky examples to get you started. If you're not on Yosemite, things work a little differently.
Customise the OS X Yosemite Login Screen [Jacob Salmela]