Important information November 2016: Due to limited server disk resources and publicly available sites which provides VBox images for free we had deleted mentioned images from our storage.
For free VBox images visit for example:
New VirtualBox BackTrack 5.1R1 appliance is available at http://nil.uniza.sk/virtualization/virtualbox/vb-images/virtualbox-images-appliances
The appliance is installed following rules described for version 5.0.