This guide will help you on how to change the login screen (the user switcher mode) apple logo to a custom image. Make sure the custom image is of size 90x90 and is in .tif format. For the best possible results, make the background of the image invisible.
1. First of all, on desktop, press 'Cmd'+'Shift'+'G' and a 'Go to Folder' window will show up. Type the following path into that window and hit enter.
2. A new finder window shows up. Holding the 'option' key on keyboard, drag the file 'applelogo.tif' to the desktop to create a backup.
3. Rename your custom image to 'applelogo.tif' and drag it to the finder window. It will now ask you to to authenticate, click 'Authenticate'. Now, it will ask you if you want to replace the current 'applelogo.tif', make sure you have followed step 2 correctly and click 'Replace'.
4. It will finally ask for your password, type it in and hit return.
Whenever you open your mac, the login screen will show the custom image instead of the plain old Apple logo.