If you’re using timthumb in your wordpress theme on WordPressMU and it’s not finding certain files, this may be why:

WordPressMU uses the blogs.dir directory to store uploads. timthumb’s search routine when looking for images to resize doesn’t include this directory.

Here a modified version I created that does include the dir:

https://markmaunder.com/timthumb.txt

You will need to add the following query param to all your timthumb.php URL’s when using this version:

blogid=<?php echo get_current_blog_id(); ?>

That way timthumb knows which blogs.dir directory to look in for images.