I used the photo from http://www.milesdavis.com (by Jeff Sedlik) and the Simple-Gnome-Logo GDM theme (by DekuHaze) to create my own GDM greeter theme:
To use it, just download GDM-Miles-Theme.tar.gz (165k) and install the theme using gdmsetup ("Graphical greeter" - "Install new theme"). Enjoy it!