Sure, that topic was deleted. The topic gave a link to an old Discourse plugin that has an example of how to register public assets in a plugin: GitHub - eviltrout/trout_emoji: Adds an evil trout emoji to a discourse install.
The last time I tested this, adding images to a plugin in the public/images directory will make them available on the server in /plugins/plugin_name/images/
The plugin name that you use in the path is the plugin name that you give in the metadata at the top of the plugin.rb file, for example /plugins/trout_emoji/images/trout-square.jpg