Example of how to create a Showdown extension to use with ember-cli-showdown for rendering Emoji images.

component.js

// ...

export default Ember.Component.extend({
  
  // ..
  showdownExtensions: ["emoji"]
  // ..
  
})

showdown-emoji.js

const emojiData = {
  "smile": "/assets/images/emoji/smile.png",
  // ...
}

showdown.extension("emoji", function() {

  return [{
    type: "lang",
    regex: ":([A-z0-9_-]+):",
    replace: function(original, emojiName) {
      if (emojiData.hasOwnProperty(emojiName)) {
        let url = emojiData[emojiName];
        return `<img src="${url}" alt="${emojiName}" class="emoji" />`;
      } else {
        // we don't know that emoji...
        return original;
      }
    }
  }];

});

template.hbs

{{markdown-to-html markdown=someContentWithEmoji extensions=showdownExtensions}}