• mogrify is part of ImageMagick, an image manipulation software suite
• mogrify manipulates the specified images. If you prefer to keep the original image untouched and write the manipulated image to a different file, simply replace mogrify with convert, the syntax is the same, but the last command line argument will be the target image to write to.
• The -geometry flag is to resize the image, it requires a dimension parameter in the format WIDTHxHEIGHT
• The dimension in this example has no width, which means the image will be resized to height=31 pixels, and the width will be proportional.

ImageMagick is not a standard package, though it is open source and available in many systems.