How to Change Login Pic for Other
One possible explanation for the lack of success so far may be that if FileVault is enabled, the system is booting from the EFI partition,(because the main drive has not yet been unlocked). 

In previous OS versions, the login screen's resources are obtained from the Recovery partition, though I mounted mine on Mohave and couldn't see anything relevant.

https://superuser.com/questions/362788/how-do-i-change-the-filevault-login-icon-on-osx-lion/364587#364587

As it turns out, replacing the login icon for the 'Other...' prompt on the login screen is as simple as:


Disable SIP.
Overwrite this file with any image of your choosing that is 72x72 pixels /System/Library/PrivateFrameworks/LoginUIKit.framework/Versions/Current/Frameworks/LoginUICore.framework/Resources/OtherUser.png