WordPress MU

I posted in the wrong forum (the Wordpress one) so I have to repost here... sorry.
I need to generate more than the usual 3 thumbs WP with fixed sizes I know (so I don't need an option page to specify size I will just hardcode them).
Do you have any suggestion on how to locate the code (and maybe some hint on wich code to add) that generates the 3 thumbs so to add 2-3 more thumbs with the sizes I need?

Thanks in advance for your kind help

Max

TimThumb.php may help You!
http://code.google.com/p/timthumb/

I'm doing this to replace the use of timthumb wich is quite cpu expensive. So I prefere to use disk space and have 6 thumbs pre-generated instead of only 3.
If in the future I will need different sizes from the old ones yes I could switch to a realtime generator, but if I can save cpu is better.
The problem is that I don't know where to look for the code wich saves the standard 3 thumbs (to add the others).
Thanks so far

You can simply change default cache time from (3600=> hour) to week or month(s) => 3600*24*30*x