manifest.json

{
  "manifest_version": 2,
  "name": "Transparent Standalone Images",
  "version": "2.0",
  "description": "This add-on renders standalone images on a transparent background, so you can see the image in all its glory!",
  "icons": {
    "64": "appearance-64.png"
  },
  "content_scripts": [{
    "matches": ["file:///*.png",  "*://*/*.png",
                "file:///*.apng", "*://*/*.apng",
                "file:///*.gif",  "*://*/*.gif",
                "file:///*.ico",  "*://*/*.ico",
                "file:///*.webp", "*://*/*.webp",
                "file:///*.jxr",  "*://*/*.jxr"],
    "css": ["transparent_image.css"]
  }]
}