This bug has been opened here: http://bugzilla.ubuntu.com/show_bug.cgi?id=16253

"click:
  System -> Administration -> Login Screen Setup

then go to the 'GTK+ Greeter' tab and click the 'Image' radio button.

then click 'Browse'.

The file browser that appears doesn't let me right-click on the right-hand pane
to get a context menu allowing me to "Show Hidden Files".  Every other GNOME
file browser I'm aware of does let me show hidden files in that way.
...
hmmm.  commenting the call to g_signal_add_emission_hook() in gdmsetup.c
restores the right-hand mouse button.

the call is commented thusly:

	/* also setup third button to work as first to work in reverse
	 * situations transparently */

so they've made the right button act like the left button, instead of like the
right button...

Wouldn't it be better if that was only done if running under gdm?  ie. include
it in the
	if (RUNNING_UNDER_GDM) { }
block that follows?"