Code:


        private void pictureBox_DoubleClick(object sender, EventArgs e)
        {
            ShowImageFullscreen(((PictureBox)sender).Image);
        }

        public void ShowImageFullscreen(System.Drawing.Image image)
        {
            Form form = new Form();
            PictureBox pictureBox = new PictureBox();
            pictureBox.Image = image;
            pictureBox.SizeMode = PictureBoxSizeMode.Zoom;
            pictureBox.Dock = DockStyle.Fill;
            form.Controls.Add(pictureBox);
            form.FormBorderStyle = FormBorderStyle.None;
            form.WindowState = FormWindowState.Maximized;
            form.KeyDown += new KeyEventHandler(form_KeyDown);
            form.ShowDialog();
        }

        public void form_KeyDown(object sender, KeyEventArgs e)
        {
            if(e.KeyCode == Keys.Escape) ((Form)sender).Close();
        }