Manual:$wgUseImageResize

Details
To disable dynamic server-side image resizing (thumbnails), set $wgUseImageResize to false.

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

If you use ImageMagick, set to true in LocalSettings.php. Make sure the variable points to the (absolute) location of the convert command of your installation and that the command is executable by the web server process.

This setting was deprecated in MediaWiki 1.10.0 and re-added in 1.15.0, it will not work in MediaWiki 1.11-1.14. In MW 1.10 and before, the default was false.