OpenSUSE 11.1 ships with GDM 2.24.x which is a not yet feature-complete rewrite of the original GDM codebase. One consequence of this is that it is no longer possible to customize the login screen using gdmsetup. It is, however, still possible to customize the background image, icon theme and Gtk theme by editing gconf settings.
The properties for GDM are defined in the distribution-specific gconf settings in /etc/gconf/gconf.xml.vendor/%gconf-tree.xml.
To change the background image, look for:
<entry name=”picture_filename” mtime=”1241629069″ type=”string”>
Change the string value to the location of the background image you want to use. This can be either a JPG file or an XML file in the Gnome background slideshow XML format (which does not seem to be documented).
The icon theme and Gtk theme can be similarly modified by editing the properties “icon_theme” and “gtk_theme” respectively.