Set lightdm user picture

I don't know if there is a way to do it with a GUI, but you could place a icon.face file in your user directory. That may cause issues, however.

An alternative is to use the AccountsService. Edit/create the file /var/lib/AccountsService/users/<username>, and add the following lines:

[User]
Icon=/somewhere/pathToIcon.icon

Make sure the lightdm user has read access to the icon (IDK, but maybe 755 permissions?)

Source: This Arch Linux wiki page.