全局处理程序

public class WaterMaker:IHttpHandler//新建一个类，实现IHttpHandler接口
{

public bool IsReusable
{
    get {
        return true;
    }
}

public void ProcessRequest(HttpContext context)
{
    context.Response.ContentType = "image/jpeg";
    //    /images/01.jpg
    string imgPath = context.Request.MapPath(context.Request.RawUrl);
    string logoPath = context.Request.MapPath("logo.png");

    using (Image img = Image.FromFile(imgPath))
    {
        using (Image logo = Image.FromFile(logoPath))
        {
            using (Graphics g = Graphics.FromImage(img))
            {
                g.DrawImage(logo, 0, 0, logo.Width, logo.Height);
                img.Save(context.Response.OutputStream, System.Drawing.Imaging.ImageFormat.Jpeg);
            }
        }
    }
}

<system.webServer>

</system.webServer>