$wgUseImageMagick = true;
$wgImageMagickConvertCommand = "/usr/bin/convert";

in your LocalSettings.php.

There are two reasons why ImageMagick is beneficial.

1. It reduces loading time for image file as it stores thumbnail image separately.
2. It gives higher quality than GD library.

However, you may encounter following error when shell memory is not allocated big enough.

Error creating thumbnail:
libgomp: Thread creation failed: Cannot allocate memory

It is due to small shell memory allocation which is handled by Mediawiki. In this case, you can simply add following in LocalSettings.php

$wgMaxShellMemory = 524288;

524288 means 512K. You can specify it as you desire. ¹