If you really want a circular effect why not override the PictureBox to behave as a Circular Picture Box. I found the code somewhere in the internet, Feel free to use it...
class OvalPictureBox : PictureBox
{
public OvalPictureBox()
{
this.BackColor = Color.Transparent;
}
protected override void OnResize(EventArgs e)
{
base.OnResize(e);
using (var gp = new GraphicsPath())
{
gp.AddEllipse(new Rectangle(0, 0, this.Width - 1, this.Height - 1));
this.Region = new Region(gp);
}
}
}