EDIT: Downloaded the image from http://tablets-dev.nokia.com/nokia_N900.php
I just hope it's the same that the updater uses (for me, the global image should be the right one). I placed it in the updaters folder, I finally found it. Since I'm an XP user, the folder path for me was:
"C:\Documents and Settings\All Users\Application Data\Nokia\Nokia Service Layer\A\nsl_service_module_00001\vpls\www.dsut.onl ine.nokia.com.oti.caresuite\Products\rx-51"

Hope it help! Updating now with NSU!
(Nokia: Get your sh*t together!)
Last edited by Larswad; Today at 05:54 PM. Reason: additions