<img src="你的一般处理程序的地址"/>

 context.Response.ContentType = "image/jpeg";

            Uri referenceUrl = context.Request.UrlReferrer;

            Uri requestUrl = context.Request.Url;

            //判断是否是本网站的请求

            if (Uri.Compare(referenceUrl, requestUrl, UriComponents.HostAndPort, UriFormat.SafeUnescaped, StringComparison.CurrentCulture) == )

            {

                context.Response.WriteFile("a.jpg");

            }

            else {

                context.Response.WriteFile("error.jpg");

            }