At Admin Panel -> Settings -> Switches there's a "post icons" option. Enabling this allows posters to select an icon which they feel expresses the overall mood of their post. The icon options are any images which in the posticons subdirectory of your images directory -- by default, /templates/images_bootstrap/smilies/ directory is used instead. The posticons subdirectory doesn't exist by default in new installations, so you'll need to create it if you want to have post icons that differ from your smilies. They'll be displayed in numerical order by file name, so name them 1.gif, 2.gif etc in your desired display order.

Post icons are common on discussion forums, and rare on other types of websites.