private void Form1_Deactivate(object sender, EventArgs e)
{
    t = new System.Threading.Timer(new System.Threading.TimerCallback(t_Tick), null, 0, 100);
}

private void Form1_Activated(object sender, EventArgs e)
{
    t.Dispose();
}

void t_Tick(object sender)
{
    pictureBox1.Invoke((Action)delegate()
    {
        pictureBox1.Update();
    });
}