C#

//cast the Image to Bitmap
Bitmap temp = (Bitmap)ActualImage;
Color c = temp.GetPixel(1,1);
//temp.SetPixel(1, 1, Color.Transparent);
temp.MakeTransparent(c);
.......
sfdlg.SaveImage(sfdlg.FileName,temp,80);

private void rename_Click(object sender, EventArgs e)
{
    DialogResult result = MessageBox.Show("Are you sure ?", "Warning", MessageBoxButtons.YesNo, MessageBoxIcon.Warning);
    if (result == DialogResult.Yes)
    {
        Thread tr = new Thread(new ThreadStart(chnTarget));
        tr.Start();
    }
}

private void chnTarget()
{
    this.Invoke(new ThreadStart(ChangeName));
}

bool cancel = false;
private void ChangeName()
{
    Random rand = new Random(DateTime.Now.Second);
    string folder_path = SelectedPathText.Text;
    long progress_value = 0;
    string new_filename = "";
    cancel = false;

    progressBar1.Minimum = 0;
    progressBar1.Value = 0;

    try
    {
        string[] FullPathFileNames = System.IO.Directory.GetFiles(folder_path);
        progressBar1.Maximum = FullPathFileNames.Length;

        if (FullPathFileNames.Length == 0) throw new Exception("No File Found \n");

        foreach (string filename in FullPathFileNames)
        {
            if (cancel)
            {
                labelResult.Text = "Canceled";
                break;
            }

            progress_value++;

            try
            {
                if (!radioRandom.Checked)
                {
                    new_filename = folder_path + "\\" + textBoxTemplate.Text + progress_value + Path.GetExtension(filename);
                }
                else
                {
                    if (checkBoxOmitExtension.Checked)
                        new_filename = folder_path + "\\" + rand.Next();
                    else
                        new_filename = folder_path + "\\" + rand.Next() + Path.GetExtension(filename);
                }

                File.Move(filename, new_filename);
                Application.DoEvents();
            }
            catch { }

            progressBar1.Value = (int)progress_value;
            progressBar1.Update();
        }

        labelResult.Text = "Complete";
        cancel = false;
    }
    catch (Exception ex)
    {
        labelResult.Text = ex.Message;
    }
}