The Beckwith book is good if you want some general info on the Tibetan military and conquests, but as for art work it is a no go, there is only one pic, and thats on the front cover, it is of men in armour though, the picture comes from a book called, A Cultural History of Tibet, by D Snellgrove and H Richardson, printed by Weidenfeld and Nicolson in 1968.
Try this site
http://tibet.prm.ox.ac.uk/tibet_collections.html or best to try any old British military manual/book from the late 19th to early 20th centuries, as remember the old saying, if a country's been invaded, the British did it first.
Or as Tony Says..try this link
http://www.amazon.com/Warriors-Himalayas-Rediscovering-Armor-Tibet/dp/0300111533