A generic error occurred in GDI+. error is occurred !!

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C#2.0

C#

Hi !!!
I am making a image upload form in asp.net c#..
and my code is for uploading is this

protected void btnupld_Click1(object sender, EventArgs e)
   {

           if (ListBox1.SelectedItem == null)
           {
               //lblmess.ForeColor = Color.Red;
               //lblmess.Text = "please select Image name to upload from box!!!";
           }
           else
           {
               if (Page.IsValid && FileUpload1.HasFile)
               {

                   string tmpname = ListBox1.SelectedItem.ToString();
                   ListBox1.Items.Insert(ListBox1.Items.IndexOf(ListBox1.SelectedItem), tmpname);
                   ListBox1.Items.RemoveAt(ListBox1.Items.IndexOf(ListBox1.SelectedItem));
                   Bitmap original_image = new Bitmap(FileUpload1.FileContent);
                   waterMarkImage(original_image, tmpname);

                   if (original_image != null)
                       original_image.Dispose();
               }
           }


   }

protected void uploadmethod(Bitmap imgBmp, string tmpName)
   {
           string newName = System.IO.Path.ChangeExtension(tmpName,"gif");
           imgBmp.Save(MapPath("~/add/" + newName));
           Image1.ImageUrl = "~/add/" + newName;

         

   }

here i changing the image extension and upload image on folder
this code is work in local machin is fine but when i upload on server i am face this error

System.Runtime.InteropServices.ExternalException: A generic error occurred in GDI+.
string newName = System.IO.Path.ChangeExtension(tmpName,"gif");
string nameBy = System.IO.Path.GetFileNameWithoutExtension(newName);
imgBmp.Save(MapPath("~/add/" + nameBy + Path.GetExtension(newName)));
Image1.ImageUrl = "~/add/" + newName;
help
thanks in advance!!!!!