First step: Prepare the picture. The only thing you need to be aware of is that if you want to get the best results, you need to prepare a picture of your screen resolution (click here) 80*95 ratio, and 160*190 proportional to replace the picture to achieve full screen replacement effect. For example, your MAC screen resolution is 1280*800, then you need to prepare a 1280*1520 resolution of the background picture (1280/80=16,16*95=1520) and 2560*3040 resolution of the picture, and all need to save as PNG format.
The second step: The two images will be given a copy, get 4 pictures. The two small resolution pictures are named Apple.png and Apple_s1.png, and two large resolution images are named Apple@2x.png and Apple_s1@2x.png respectively.
Step three: Then press the shortcut key command+shift+g in the Finder to enter the following path:/system/library/privateframeworks/loginuikit.framework/versions/a/ frameworks/loginuicore.framework/resources/
Step Fourth: Locate the Apple.png, Apple_s1.png, Apple@2x.png, apple_s1@2x.png four pictures in the folder, back them up to a safe location, and replace them with the 4 pictures that you got before (you need to enter an Administrator password Authentication). After you log off the current account, you can see in the login interface that the new background is in effect.