Go to the Users & Groups System Preferences/Settings app to see if the icon still shows the problem.
What is the exact model of this Mac and version of macOS? You can get this information by clicking the Apple menu and selecting "About This Mac". If this is an Intel Mac, then is Filevault enabled?
Has this icon/picture displayed properly before this? If so, what has recently changed on your Mac? Did you install a macOS update patch? Or upgraded macOS to a new major version when this issue occurred?
Is this a custom picture icon for login or one of the macOS defaults provided by macOS? If it is a custom picture or icon you created or downloaded, then it sounds like it may be an incompatible picture format. I'm not sure what picture format macOS prefers for a login icon.