To add custom flags to your server add a .png file into the /AdvancedFlags directory in the /plugins folder. The image should be small as large images slow down the server and make the ping horrible. The image name should be 2 letters and should not be taken by a country (see https://en.wikipedia.org/wiki/ISO_3166-1_alpha-2). If you encounter an error report it on the Github page at https://github.com/mrxbox98/AdvancedFlags