Create a shape image in C# with transparent surroundings

I want to create an image in C#, red ellipse with transparent surroundings.  What do I need to add to the code below to get the area  around my ellipse to be transparent.  I will later use this image on top of another image.

System.Drawing.Bitmap bitmap = new System.Drawing.Bitmap(100, 100);

Graphics g = Graphics.FromImage(bitmap);

SolidBrush mySolidBrush = new SolidBrush(System.Drawing.Color.Red);

g.FillEllipse(mySolidBrush, 0, 0, 100, 100);

bitmap.Save(@"C:\test\test.jpeg", ImageFormat.Jpeg);
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Mike TomlinsonHigh School Computer Science, Computer Applications, and Mathematics TeachersCommented:
What do I need to add to the code below to get the area  around my ellipse to be transparent.

First, you cannot use Jpeg for transparent regions; use PNG or GIF.  The Bitmap.MakeTransparent() method will be helpful here:
            System.Drawing.Bitmap bitmap = new System.Drawing.Bitmap(100, 100);
            using (Graphics g = Graphics.FromImage(bitmap))
                g.FillEllipse(Brushes.Red, 0, 0, 100, 100);
            bitmap.Save(@"C:\test\test.png", System.Drawing.Imaging.ImageFormat.Png);

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