Manual:$wgDjvuPostProcessor

Details
Path of the DjVu post-processor. May include command line options. The output generated by $wgDjvuRenderer is passed onto this program.

The default:  is used, since ddjvu generates ppm output, which needs to be converted to JPEG to be viewed in browsers.

If your post-processor outputs anything other than .jpg files, you need to set $wgDjvuOutputExtension as well.