_load_images() {
    if (
      this.options.shape === "circularImage" ||
      this.options.shape === "image"
    ) {
      if (this.options.image === undefined) {
        throw new Error(
          "Option image must be defined for node type '" +
            this.options.shape +
            "'"
        );
      }
    }

    if (this.options.image === undefined) {
      return;
    }

    if (this.imagelist === undefined) {
      throw new Error("Internal Error: No images provided");
    }

    if (typeof this.options.image === "string") {
      this.imageObj = this.imagelist.load(
        this.options.image,
        this.options.brokenImage,
        this.id
      );
    } else {
      if (this.options.image.unselected === undefined) {
        throw new Error("No unselected image provided");
      }

      this.imageObj = this.imagelist.load(
        this.options.image.unselected,
        this.options.brokenImage,
        this.id
      );

      if (this.options.image.selected !== undefined) {
        this.imageObjAlt = this.imagelist.load(
          this.options.image.selected,
          this.options.brokenImage,
          this.id
        );
      } else {
        this.imageObjAlt = undefined;
      }
    }
  }
Load the images from the options, for the nodes that need them.

Images are always loaded, even if they are not used in the current shape.
The user may switch to an image shape later on.
@private