Examine source code of Focus Gallery

{
  "description": "Create a gallery of all images from a webpage",
  "manifest_version": 2,
  "name": "Focus Gallery",
  "version": "1.0.7",
  "icons": {
    "32": "32.png",
    "48": "48.png"
  },
  "background": {
    "scripts": [
      "background.js"
    ]
  },
  "permissions": [
    "tabs",
    "http://*/*",
    "https://*/*",
    "activeTab",
    "downloads",
    "storage",
    "menus"
  ],
  "commands": {
    "start-gallery": {
      "suggested_key": {
        "default": "Alt+G"
      },
      "description": "Gallery for current page, or selection"
    }
  },
  "browser_specific_settings": {
    "gecko": {
      "id": "{596b3310-f21c-41a2-99b1-76043b363ed2}"
    }
  }
}