multisite images not showing up

I’m running a multisite install and the images on my site suddenly stopped showing up; I think I’ve traced it to a .htaccess issue but now I’m stuck.

Using the default .htaccess code displays only broken images on my site:

# BEGIN WordPress

<IfModule mod_rewrite.c>

RewriteEngine On
RewriteBase /
RewriteRule ^index\.php$ - [L]

# uploaded files
RewriteRule ^([_0-9a-zA-Z-]+/)?files/(.+) wp-includes/ms-files.php?file=$2 [L]

# add a trailing slash to /wp-admin
RewriteRule ^([_0-9a-zA-Z-]+/)?wp-admin$ $1wp-admin/ [R=301,L]

RewriteCond %{REQUEST_FILENAME} -f [OR]
RewriteCond %{REQUEST_FILENAME} -d
RewriteRule ^ - [L]
RewriteRule ^([_0-9a-zA-Z-]+/)?(wp-(content|admin|includes).*) $2 [L]
RewriteRule ^([_0-9a-zA-Z-]+/)?(.*\.php)$ $2 [L]
RewriteRule . index.php [L]
</IfModule>

# END WordPress

… but if I replace the RewriteRule ^([_0-9a-zA-Z-]+/)?files/(.+) wp-includes/ms-files.php?file=$2 [L] line with this, it ALMOST works:

RewriteRule ^([_0-9a-zA-Z-]+/)?files/(.+) wp-content/blogs.dir/3/files/$2 [L]

… the only trouble is, my theme uses timthumb pretty extensively and it’s still showing broken images through timthumb and I don’t particularly want to edit the .htaccess file for every site I maintain. How can I fix this?