Manual:$wgGenerateThumbnailOnParse

Details
Allow thumbnail rendering on page view. If this is false, a valid thumbnail URL is still output, but no file will be created at the target location. This may save some time if you have a thumb.php or 404 handler set up which is faster than the regular webserver(s).

Setting this to true is equivalent to setting the  property in $wgLocalFileRepo and $wgForeignFileRepos to false.