Manual:Thumb handler.php/en

Description
thumb_handler.php is a script to be used to automatically resize images from a 404 handler. For example, when a browser requests a thumbnail that has not been created before.

To use it, follow the steps below, then set  to false. If you have  defined in LocalSettings.php, then you need to also set:

Apache
Create a rewrite rule to call thumb_handler.php when a file in /thumb/ doesn't exist. If your wiki is in the /w directory, something like this should work on apache: