By default, background image (wallpaper, let's call it that) a login screen on OS X, is an unclear picture, blurred, active screen, or wallpaper set on Desktop.
The screenshot above is background image of the Desktop that appears blurred in the Login Screen background.
OS X users can easily change this image.
First of all, we will have to decide and choose a picture we want to set as a wallpaper on the login screen. After we decide on an image, it will have to be in .PNG format and have the name: “com.apple.desktop.admin.png". If our image is in .JPG format, we open the image in the Preview app, We go to the top menu at "Fillet”>“Export”And select format . PNG.
After we have the saved image, copy it to the folder: /Library/Caches/. It is good to rename the new image in "Caches" before copying the original file "Com.apple.desktop.admin.png " in "com.apple.desktop.admin-backup. Png".
The result:
This tutorial is only for OS X Yosemite si OS X El Capitan. Possible for future versions of OS X.
You can do screenshot of the login screen (login screen), following this tutorial.