I know this is a quite stupid question but I cannot find the solution.
I would like to change the user picture that is displayed when GDM is loaded.
I read that for Ubuntu you only need to go in System/aboutMe. In xfce I don't have this menu.

Thank you for the replies.

P.S. I think believe that also put a jpg somewhere like ~/.profilepicturegdm2... would work but I don't know the right directory in this case.