yuopi

System.Drawing.Drawing2D.GraphicsPath gp = new System.Drawing.Drawing2D.GraphicsPath();
            gp.AddEllipse(0, 0, pictureBox1.Width - 3, pictureBox1.Height - 3);
            Region rg = new Region(gp);
            pictureBox1.Region = rg; //pictureBox1 - ваш бокс