Manual:$wgCustomConvertCommand

Details
To use another resizing converter, e.g. GraphicMagick, set this variable to the command-line string required to launch the executable. If no string is specified then MediaWiki's internal resizing code will be used.

In the command-line string, the following variables will be expanded:
 * %s will be replaced with the source path
 * %d will be replaced with the destination
 * %w will be replaced with the width
 * %h will be replaced with the height

For example, here is the command you would set for GraphicMagick: