Comment 6 for bug 1086199

This patch works for me

--- lightdm/src/lightdm-gtk-greeter.c 2012-08-30 18:39:09.000000000 +0100
+++ lightdm/src/lightdm-gtk-greeter.c 2012-12-20 18:30:03.216355000 +0000
@@ -801,6 +801,7 @@
            g_warning ("Failed to load background: %s", error->message);
         g_clear_error (&error);
         g_free (path);
+ background_pixbuf=gdk_pixbuf_add_alpha (background_pixbuf,FALSE,255,255,255);
     }
     else
         g_debug ("Using background color %s", value);

If the background file does not have an alpha channel, then it will add one.