向高手请教一个关于导出到Word的问题

Question

我用下面这种方式将页面导出到Word时背景图片导不出来是怎么回事？ 
string fileName = string.Empty;
            if (string.IsNullOrEmpty(fileName))
            {
                fileName = "著作工作量分配单";
            }
            System.IO.StringWriter sw = new System.IO.StringWriter();
            HtmlTextWriter htw = new HtmlTextWriter(sw);
            this.Page.Form.RenderControl(htw);
            string pageHtml = sw.ToString();
            int startIndex = pageHtml.IndexOf("<div id=\"Print\"");
            int endIndex = pageHtml.LastIndexOf("</div>");
            int lenth = endIndex - startIndex;
            pageHtml = pageHtml.Substring(startIndex, lenth);
            pageHtml = pageHtml.Remove(pageHtml.LastIndexOf("<div>"));
            HttpContext context = HttpContext.Current;
            context.Response.AppendHeader("Content-Disposition", "attachment;filename=" + System.Web.HttpUtility.UrlEncode(fileName, System.Text.Encoding.UTF8) + ".doc");
            context.Response.ContentType = "application/ms-word";
            context.Response.ContentEncoding = System.Text.Encoding.UTF8;
            context.Response.Charset = "UTF8";
            context.Response.Write(pageHtml);
            context.Response.End();