I have shown you how to modify the login window for Mavericks and Yosemite.  The Yosemite trick still works in El Capitan with one added benefit: there is no longer a blurred overlaid on your image.

Just save your image as (it needs to a .png file):

/Library/Caches/com.apple.desktop.admin.png

Then log out and you should see your image.  It still works best to have your image right or left-justified so that it doesn’t cover up the username and password fields.