In the release notes is mentioned that the gthumb image viewer package has been added to the distro, however it is not installed by default and is not present among the list of packages which can be selected during the installation (or launching redhat-config-packages). Maybe it should substitute GQview which is another gtk image viewer but still based on gtk-1.2.
It is installed by default in our current trees if you perform a gnome-desktop install. I'm closing this as rawhide since it's working for us internally.