vega with conditional image (centered and stretched to the largest size preserving aspect ration)

vega_img.json

{
  "$schema": "https://vega.github.io/schema/vega/v4.json",
  "width": 200,
  "height": 200,
  "padding": 5,
  "marks": [
    {
      "type": "image",
      "encode": {
        "enter": {
          "url": [
            {
              "test": "false",
              "value": "https://www.maxpixel.net/static/photo/1x/Ailurus-Fulgens-Red-Panda-Panda-Fire-Fox-Bear-Cat-3040830.jpg"
            },
            {
              "value": "https://www.thelocal.de/userdata/images/article/fa6fd5014ccbd8f4392f716473ab6ff354f871505d9128820bbb0461cce1d645.jpg"
            }
          ],
          "x": {"signal": "width/2"},
          "y": {"signal": "height/2"},
          "width": {"signal": "width"},
          "height": {"signal": "height"},
          "align": {"value": "center"},
          "baseline": {"value": "middle"}
        }
      }
    }
  ]
}