Critter,
This is an answer I gave a while back:
In PPT 2002/2003 but something similar is possible in PPT 2000.
View - master - slidemaster - Insert - Picture - from file - select your
picture - right click it - choose Format picture - picture tab - colour
list - choose watermark - OK. With the picture still selected consider
alignment and size, if all is well then rightclick the picture again choose
order and select to background. Leave your slidemaster and you are all set.
Luc