文件操作（增删查改）

//url为图片的相对路径(~/UpLoadFiles/test.jpg)，path为将url转换为绝对路径，通过File类操作
string path = HttpContext.Current.Server.MapPath(url);
 if (File.Exists(path))
 {
     File.Delete(path);
 }

//新建bmp图片
//W,H为宽高，像素为单位
//path为相对路径(~/UpLoadFiles/t.jpg)                                          
System.Drawing.Bitmap timage = new System.Drawing.Bitmap(W, H);
try
{
    //缩图图保存
    timage.Save(path, System.Drawing.Imaging.ImageFormat.Jpeg);
}
catch (Exception ex)
{
    throw ex;
}
finally
{
    timage.Dispose();
}