private void btnGuardaImatge_Click(object sender, EventArgs e)
    {
        using (SaveFileDialog saveFileDialog = new SaveFileDialog())
        {
            saveFileDialog.InitialDirectory = "c:\";
            saveFileDialog.Filter = "Bitmap (*.bmp)|*.bmp| Png| *.png| Tots els fitxers(*.*)|*.*";
            saveFileDialog.FilterIndex = 1;

            if (saveFileDialog.ShowDialog() == DialogResult.OK)
            {
                Bitmap b = new Bitmap(saveFileDialog.FileName);
                pctOriginal.Image = b;
                this.imatge = b;
            }
        }
    }

Bitmap b = new Bitmap(Filename: saveFileDialog.FileName);