How can I make a tab separator Chrome extension?

I am currently trying to make a Google Chrome extension that makes a new divider when you click the icon. Currently, I am simply making the extension open a new tab when the icon is pressed, how can I change this to be able to add an image in the task bar.

Manifest.json

{
  "manifest_version": 2,

  "name": "Moses - Part the Tab Sea",
  "description": "This extension adds the ability to add separators to 
   tabs.",
  "version": "1.0",

  "browser_action": {
     "default_icon": "icon.png",
     "default_popup": "popup.html",
     "default_title": "Add a Divider"
  },

  "background": {
     "scripts": ["eventPage.js"],
     "persistent": false
  },

  "permissions": [
     "tabs"
  ]
}

Popup.js

document.onclick = addDivider();

function addDivider() {
    chrome.tabs.create({index:(15)});
}