Originally Posted by keenart
Try Paint.net a free software program that has posterizing. It is up to date, unlike Gimp which is old stuff and has not been supported for many years now.
I'm not trying to start a debate here, but I have no idea where you're getting your information regarding GIMP. Before making a comment 'that it has not been supported for many years now' you may want to first check the GIMP website (gimp.org) where you will find that version 2.8.20 was released Feb. 1, 2017, that macOS and Windows upgrades were released on Feb. 7, 2017, and additional development plans are underway for major improvements for a 2.8.20 release sometime in the future.
I have no affiliation with GIMP, but being a software developer I have a lot of respect for a development community that offers a FREE open-source application that easily rivals some of the best commercial graphic programs available.