message: add img path
diff: @@ -7,6 +7,7 @@ import Billboard from "../Billboard";
  import BillboardCollection from "../BillboardCollection";
  const center = Cartesian3.fromDegrees(-75.59777, 40.03883);
  +import exampleImg from "./assets/example.jpg";
  export default () => {
  storiesOf("Billboard", module).add("default", () => (
  @@ -15,22 +16,22 @@ export default () => {
  <Billboard
  color={Color.ORANGE}
  position={new Cartesian3(0.0, 0.0, 0.0)}
  - image="assets/example.jpg"
  + image={exampleImg}
  />
  <Billboard
  color={Color.YELLOW}
  position={new Cartesian3(1000000.0, 0.0, 0.0)}
  - image="assets/example.jpg"
  + image={exampleImg}
  />
  <Billboard
  color={Color.GREEN}
  position={new Cartesian3(0.0, 1000000.0, 0.0)}
  - image="assets/example.jpg"
  + image={exampleImg}
  />
  <Billboard
  color={Color.CYAN}
  position={new Cartesian3(0.0, 0.0, 1000000.0)}
  - image="assets/example.jpg"
  + image={exampleImg}
  />
  </BillboardCollection>
  </Viewer>
  