Convert the image to grayscale

Perform lossless transformation of JPEG files.
More information: <https://manned.org/jpegtran>.
Trick copyright: tl;dr; <https://github.com/tldr-pages/tldr>

jpegtran -grayscale {{path/to/image.jpg}} > {{path/to/output.jpg}}

click the source code to copy install jpegtran on any operating system with command-not-found.com