public static void ShowImage(string imageUrl, System.Web.UI.WebControls.Image image1, int width, int height)
    {
        string Url = imageUrl;
        try
        {
            System.Drawing.Image image = System.Drawing.Image.FromFile(HttpContext.Current.Server.MapPath(Url));
            System.Drawing.Size size = new System.Drawing.Size(image.Width, image.Height);
            int w = size.Width;
            int h = size.Height;
            if (w > h)
            {
                if (w > width)
                {
                    w = width;
                    h = (w * size.Height) / size.Width;
                }
            }
            else
            {
                if (h > height)
                {
                    h = height;
                    w = (h * size.Width) / size.Height;
                }
            }

            image1.Attributes.Add("style", "width:" + w + "px;height:" + h + "px;");
            image1.ImageUrl = Url;
        }
        catch
        {
            image1.ImageUrl = Url;
        }
    }

------解决方案--------------------
先正常加载，然后cs里 用楼上的方法，循环缩放。可以吗