Two things: firstly, if your images are large, then it's better to not store them in the DB - use a file and store the path to the file in the DB instead.
Second, your code tells it to store the large image twice because you do nothing with the thumbnail image once you have created it, just add the ms buffer twice. You need to save the new thumbnail image to the
thumb
stream and fetch that buffer before you set the parameter value.
Your code would also be a little more obvious if you used "proper" names fro teh SQL parameters, and AddWithValue instead of Add:
string cb = "insert into Customer(C_ID,CustomerID,Name,Address,City,ContactNo,Email,Photo,Thumbnail) VALUES (@ID,@CID,@NAM,@ADDR,@CITY,@CNO,@EML,@IMAGE,@THUMB)";
cc.cmd = new SqlCommand(cb);
...
byte[] data = ms.GetBuffer();
cc.cmd.Parameters.AddWithValue("@IMAGE", data);
...
byte[] thumbnail = thumb.GetBuffer();
cc.cmd.Parameters.AddWithValue("@THUMB", thumbnail);
...