Manual:$wgUseImageResize

Details
Set $wgUseImageResize to true if you want to enable dynamic server-side image resizing ("Thumbnails").

This parameter requires a functioning version of PHP's GD library or ImageMagick to take effect. It has effect if no image processing tool is configured. Enabling resizing may generate error messages on pages with thumbnails if these components are improperly configured.

To enable image thumbnailing, set $wgUseImageResize (and $wgUseImageMagick if you want to use it) to true in LocalSettings.php. Make sure the $wgImageMagickConvertCommand variable points to the proper location of the convert command of your installation and that the command is executable by the web server process.

If both ImageMagick and GD library are installed, MediaWiki will use GD library. To change this, edit LocalSettings.php after installation and change $wgImageMagickConvertCommand from "/usr/bin/convert"; to "/usr/local/bin/convert";.