Night Watch (Terry Pratchett) Easter Egg - Night Watch Cover

Not actually by Terry himself, but on the back cover of Night Watch you will see the Rembrandt painting Nightwatch, which Paul Kidby's front cover art is based on.

Look up the Rembrandt picture in the eeggs database and you'll see that Rembrandt painted himself into the picture. Look in the same place on Paul Kidby's cover (just above Reg Shoe's shoulder) and you will see Josh Kirby, famous for his cover arts on previous Discworld novels, who died recently.