The default user icon for the SDDM theme does not fit into the Breeze style
we have the avatars from jens maybe they can work. maybe he can design an guest avatar too.
the default sddm icon was used it's located in /usr/share/sddm/faces/.face.icon
the sddm theme had to be changed to an defined Avatar.
David or marcho can help us.
This was done, by you, in D11057! :)