logon display icons always in alphabetical order

On my login screen the login names are in alphabetical order but i want one to be selected as a default automatically. it tries to use the top or bottom one. how can i force it to use a specific one in the list? I've have searched google with no success.

Many thanks, Errol Greer

I have two logon names, my first name and Dad. They are listed in order of the last one I used and that's the one that appears by default next time I go to log on.