The gnome-base/gnome ebuild appears to require media-libs/gdk-pixbuf in order to display any sort of graphics in the GNOME desktop, but doesn't emerge it as a dependency.
Or, possibly, as has been pointed out on gentoo-user, gdk-pixbuf is emerged before PNG support.
might very well be that this was changed in gnome-core so that it now uses gdk-pixbuf instead of imlib.
added gdk-pixbuf to deps in gnome-core