I'm not sure if this helps but I think this is closely related to upsteam bug:
http://bugzilla.gnome.org/show_bug.cgi?id=321840 -- In slide show mode gthumb always uses original size https://launchpad.net/bugs/47070 -- ubuntu-bugs mailing list ubuntu-bugs@lists.ubuntu.com https://lists.ubuntu.com/mailman/listinfo/ubuntu-bugs
