LocalFileRemoteFallbackImageService.cs

using System;
using System.IO;
using System.Net;
using System.Threading.Tasks;
using System.Web;
using System.Web.Hosting;
using ImageProcessor.Web.Services;

namespace UmbrellaInc.Web.Services
{
    public class LocalFileRemoteFallbackImageService : LocalFileImageService
    {
        public override async Task<byte[]> GetImage(object id)
        {
            try
            {
                return await base.GetImage(id);
            }
            catch (HttpException ex)
            {
                if (ex.GetHttpCode() != 404)
                {
                    throw ex;
                }
            }

            var path = (string)id;
            var host = this.Settings["Host"];
            var relativePath = path.Replace(HostingEnvironment.ApplicationPhysicalPath, string.Empty);
            var uri = new Uri(new Uri(host), relativePath);

            var dir = Path.GetDirectoryName(path);
            if (Directory.Exists(dir) == false)
            {
                Directory.CreateDirectory(dir);
            }

            using (var client = new WebClient())
            {
                if (uri.Scheme == Uri.UriSchemeHttps && ServicePointManager.SecurityProtocol.HasFlag(SecurityProtocolType.Tls12) == false)
                {
                    ServicePointManager.SecurityProtocol = ServicePointManager.SecurityProtocol | SecurityProtocolType.Tls12;
                }

                client.DownloadFile(uri, path);
            }

            return await base.GetImage(id);
        }
    }
}

security.config

<?xml version="1.0" encoding="utf-8"?>
<security>
  <services>
    <service name="LocalFileRemoteFallbackImageService" type="UmbrellaInc.Web.Services.LocalFileRemoteFallbackImageService, UmbrellaInc.Web">
      <settings>
        <setting key="Host" value="https://yourhost.com/" />
      </settings>
    </service>
  </services>
</security>