try this,
if (hdnImageValues.Value != "")
{
string filePath = @"D:\Images\" + hdnImageValues.Value;
string fileName = hdnImageValues.Value;
if (File.Exists(filePath))
{
FileStream fs = File.Open(filePath, FileMode.Open);
byte[] fsbyte = null;
fsbyte = new byte[fs.Length];
fs.Read(fsbyte, 0, Convert.ToInt32(fs.Length, null));
fs.Close();
Response.AddHeader("contentdisposition","attachment;filename=" + fileName);
Response.BinaryWrite(fsbyte);
Response.End();
}
}