how to save a bitmap and upload it in editable mode?

Question

im doing a painter project in c# in windows forms application and im trying to save the drawing and upload it the picturebox . the image is load, but im not able to keep working on it . is there a way to save and load the a bitmap in 'mdl' format or something else so i could keep working on it ?

this is how i did it :

   private void btn_save_Click(object sender, EventArgs e)
    {
        SaveFileDialog saveFileDialog1 = new SaveFileDialog();
        saveFileDialog1.InitialDirectory = Directory.GetCurrentDirectory();// + "..\\myModels";
        saveFileDialog1.Filter = "Images (.bmp;.jpg;.png)|.bmp;.jpg;.png";
        saveFileDialog1.FilterIndex = 1;
        saveFileDialog1.RestoreDirectory = true;
        if (saveFileDialog1.ShowDialog() == System.Windows.Forms.DialogResult.OK)
        {
            bm.Save(saveFileDialog1.FileName, ImageFormat.Jpeg);
        } 
    }

    private void btn_Load_Click(object sender, EventArgs e)
    {

        OpenFileDialog open = new OpenFileDialog();
                   
        open.Filter = "Image Files(.jpg; *.jpeg; *.gif; *.bmp)|.jpg; *.jpeg; *.gif; *.bmp";
        if (open.ShowDialog() == DialogResult.OK)
        {
 
            pic.Image = new Bitmap(open.FileName);
            pic.Refresh();
        }
    }