defaultValue for propType imageUrl

Just wondering how can i register a component with a prop of type imageUrl and supply a default preview image?

  props: {
    image: {
      type: "imageUrl",
      defaultValue: {
        type: "img",
        src: ""

This i thought would be the obvious but does not work. The error i get: Type '"imageUrl"' is not assignable to type '"custom"