private void histogramToolStripMenuItem1_Click(object sender, EventArgs e)
 {
     if (_image != null)
     {
         HistogramForm form = new HistogramForm(_image);
         form.Show();
     }
     else
     {
         MessageBox.Show("Select image first.");
     }
 }
        private void button2_Click(object sender, EventArgs e)
        {
            HistogramForm form = new HistogramForm(formImage);

            form.Show();
        }