Vega tooltip with images

spec.json

{
  "$schema": "https://vega.github.io/schema/vega/v5.json",
  "padding": 5,
  "height": 100,
  "data": [
    {
      "name": "data",
      "values": [
        {"img": "data/ffox.png"},
        {"img": "data/gimp.png"},
        {"img": "data/7zip.png"}
      ]
    }
  ],
  "marks": [
    {
      "name": "marks",
      "type": "text",
      "style": ["text"],
      "from": {"data": "data"},
      "encode": {
        "update": {
          "y": {"scale": "y", "field": "img"},
          "text": {"field": "img"},
          "tooltip": {
            "signal": "{title: datum.img, image: datum.img}"
          }
        }
      }
    }
  ],
  "scales": [
    {
      "name": "y",
      "type": "point",
      "domain": {"data": "data", "field": "img", "sort": true},
      "range": [0, {"signal": "height"}]
    }
  ]
}