The painting is called the Meagre Company, or The Company of Captain Reinier Reael and Lieutenant Cornelis Michielsz Blaeuw, painted by Franz Hals in 1633 and completed by Pieter Codde when Hals got sick of the commute to Amsterdam (seriously).
That makes it a few years younger than the famous and somewhat similar 1642 Night Watch by Rembrandt, which was a partial inspiration for Pratchett's book.