So, on my dev wiki, instantcommons thumbnails don't work. I have rewrites setup to use thumb.php
RewriteCond %{REQUEST_FILENAME} !-f
RewriteCond %{REQUEST_FILENAME} !-d
RewriteRule ^/w/images/thumb/[0-9a-f]/[0-9a-f][0-9a-f]/([^/]+)/([0-9]+)px-.*$ /w/thumb.php?f=$1&width=$2 [L,QSA,B]
RewriteCond %{REQUEST_FILENAME} !-f
RewriteCond %{REQUEST_FILENAME} !-d
RewriteRule ^/w/images/thumb/archive/[0-9a-f]/[0-9a-f][0-9a-f]/([^/]+)/([0-9]+)px-.*$ /w/thumb.php?f=$1&width=$2&archived=1 [L,QSA,B]
# Proofread Page
RewriteCond %{REQUEST_FILENAME} !-f
RewriteCond %{REQUEST_FILENAME} !-d
RewriteRule ^/w/images/thumb/[0-9a-f]/[0-9a-f][0-9a-f]/([^/]+)/page([0-9]+)-?([0-9]+)px-.*$ /w/thumb.php?f=$1&page=$2&width=$3 [L,QSA]Thumbnails exist on disk
reedy@ubuntu64-web-esxi:/var/www/wiki/mediawiki/core/images/thumb/e/e6/Mantelaffe-mit-Baby-Zoo-Muenster.jpg$ ls -al total 9400 drwxr-xr-x 2 www-data www-data 4096 Oct 15 16:38 . drwxr-xr-x 3 www-data www-data 4096 Oct 15 16:37 .. -rw-r--r-- 1 www-data www-data 15288 Oct 15 16:37 159px-Mantelaffe-mit-Baby-Zoo-Muenster.jpg -rw-r--r-- 1 www-data www-data 65691 Oct 15 16:37 397px-Mantelaffe-mit-Baby-Zoo-Muenster.jpg -rw-r--r-- 1 www-data www-data 135923 Oct 15 16:37 595px-Mantelaffe-mit-Baby-Zoo-Muenster.jpg -rw-r--r-- 1 www-data www-data 237324 Oct 15 16:38 793px-Mantelaffe-mit-Baby-Zoo-Muenster.jpg -rw-r--r-- 1 www-data www-data 7302 Oct 15 16:38 79px-Mantelaffe-mit-Baby-Zoo-Muenster.jpg -rw-r--r-- 1 www-data www-data 9143824 Oct 15 16:37 Mantelaffe-mit-Baby-Zoo-Muenster.jpg reedy@ubuntu64-web-esxi:/var/www/wiki/mediawiki/core/images/thumb/e/e6/Mantelaffe-mit-Baby-Zoo-Muenster.jpg$
Visiting http://10.0.0.190/w/images/thumb/e/e6/Mantelaffe-mit-Baby-Zoo-Muenster.jpg/397px-Mantelaffe-mit-Baby-Zoo-Muenster.jpg gives
The source file 'Mantelaffe-mit-Baby-Zoo-Muenster.jpg' does not exist.