Adding custom image preview for a document in Sanity

// language.jsx

import React from "react";

const schema = {
  name: "language",
  type: "document",
  title: "Language",
  fields: [
    {
      name: "title",
      type: "string",
      title: "Title",
      description: "For example English (UK), Norwegian"
    },
    {
      name: "localeTitle",
      type: "string",
      title: "Locale title",
      description: "For example Norsk, Svenska"
    },
    {
      name: "localeId",
      type: "string",
      title: "Locale ID",
      description: "For example en-US"
    }
  ],
  preview: {
    select: {
      title: "title",
      locale: "localeId"
    },
    prepare({ title, locale }) {
      return {
        title: title,
        subtitle: locale,
        media: <img src="<https://purecatamphetamine.github.io/country-flag-icons/3x2/US.svg>" alt={`${title}`} />
      };
    }
  }
};

export default schema;