Chrome Extension Icon Manifest

It should be:

{
  "manifest_version": 2,
  "name": "Demo",
  "description": "This is demo.",
  "version": "1.0",
  "icons": {
    "16": "icon16.png",
    "48": "icon48.png",
    "128": "icon128.png"
  },
  "browser_action": {
    "default_popup": "popup.html"
  },
  "permissions": ["activeTab", "storage"]
}  

See: Chrome Extension Manifest Docs

Set "icons" key in manifest.json.

The browser_action.icons key is what gets displayed in the toolbar (and will probably only use 16, 24, and 32 sized images, see browserAction).

The one displayed in chrome://extensions is a top level icons key. In the manifest documentation, look for the 6th entry, so that your manifest has an top-level entry, like:

{
  "icons": {
    "16": "icon16.png",
    "48": "icon48.png",
    "128": "icon128.png"
  }
}