How do you insert a image file into a JSON code on sharepoint online list

We can add iconName attribute as shown below:

{
  "$schema": "https://developer.microsoft.com/json-schemas/sp/v2/column-formatting.schema.json",
  "elmType": "div",
  "style": {
    "font-size": "12px"
  },
   "attributes": {
    "iconName": "SunQuestionMark"
  },
  "customCardProps": {
    "formatter": {
      "elmType": "div",
      "txtContent": "[$Description]",
      "style": {
        "fontsize": "12px",
        "color": "green",
        "padding": "5px"
      }
    },
    "openOnEvent": "hover",
    "directionalHint": "bottomCenter",
    "isBeakVisible": true,
    "beakStyle": {
      "backgroundColor": "white"
    }
  }
}

My test result for your reference: