In OS X Yosemite your login screen is usually made from currently set wallpaper mixed with a blur effect. This version of the OS gives you an easy way to change this picture to something completely different.
All you have to do is to prepare the image you want to use and save it as a PNG file.
Save your PNG at:
Now simply log out to see the effect immediately.
Pretty easy stuff!
Side Note 🪳 🍎
Unfortunately in further version of OS X / macOS, this solution seems to not work anymore, as the
com.apple.desktop.admin.png does not exist.