You can draw an Ellipse on the pictureBox image using the following code:

public void drawEllipse(PictureBox pb, int x, int y, int w, int h, float Bwidth,Color col)
        {
            //refresh the picture box
            pb.Refresh();
            //create a graphics object
            Graphics g = pb.CreateGraphics();
            //create a pen object
            Pen p = new Pen(col, Bwidth);
            //draw Ellipse
            g.DrawEllipse(p, x, y, w, h);
            //dispose pen and graphics object
            p.Dispose();
            g.Dispose();
        }

In here, I grab the picturebox graphics using pb.CreateGraphics() so that I can draw on its surface. But drawing on the PictureBox only affects the pixels on the screen, not the pixels in the image. You can however, obtain the Graphics for the image. You can see that in the next example.

You can draw a line on the pictureBox image using the following code:

public void drawline(PictureBox pb, Point p1, Point p2, float Bwidth,Color c1)
        {
            //refresh the picture box
            pb.Refresh();
            //create a new Bitmap object
            Bitmap map=(Bitmap)pb.Image;
            //create a graphics object
            Graphics g = Graphics.FromImage(map);
            //create a pen object and setting the color and width for the pen
            Pen p = new Pen(c1, Bwidth);
            //draw line between  point p1 and p2
            g.DrawLine(p, p1, p2);
            pb.Image = map;
            //dispose pen and graphics object
            p.Dispose();
            g.Dispose();
        }

After creating the graphics object(g), there are several predefined methods you can use to draw objects. Some are listed below:

g.DrawArc
g.DrawCurve
g.DrawBezier
g.DrawIcon
g.DrawImage
g.DrawPath
g.DrawRectangle