There's an error when using the TimThumb script and WordPress MU. If you link to images hosted on one of your other blogs, they will not show. WARNING: We strongly recommend waiting for TimThumb to release a fix.
However, some people are making unofficial changes to TimThumb and having success. You can read about those at these resources:
mu.wordpress.org 1
mu.wordpress.org 2
mu.wordpress.org 3
mu.wordpress.org 4
