Not sure what you actually asking but here are some links

FIASCO (image format), acronym of "Fractal Image And Sequence Codec"

http://wiki.maemo.org/Flasher
Scroll halfway down for more nfo in that wiki.

http://wiki.maemo.org/Updating_the_tablet_firmware

More nfo about update.

Not sure if this is what your looking for, but hope it helps!

*edit

If you have a N900 that reboot every-now-and-then, here´s an advice: SEND IT BACK ASAP to be replaced.

I had 100% working pre-release N900 to play with and got it exchanged for a retail version, the retail had the rebooting endlessly sometimes and sometimes 5 min sometimes longer.
No matter what I did with reflashing or followed the bugzill discussion helped much, just lowered the number of reboots.

Handed it back to Nokia and got my 2nd retail N900. ROCK SOLID.

So no use in flashing it unless you screwed with firmware and it was working fine before If it´s been rebooting from day 1 just send it back to be replaced.

That´s my advice.