I WANT TO DRAW A CIRCLE ON A IMAGE

Question

SIR,
     i am doing a application for immage processing where i am comparing 2 immages,,and i want to higlight the pixcles where the diffrence occours with a red colored circle..i am able to compare the images but not able to higlight it with the circle..i want the pixel which is diffrent to ve the center of the circle..
 
if (img1.Width == img2.Width && img1.Height == img2.Height)
{
//drawing
g = pictureBox1.CreateGraphics();
//////g = Graphics.FromImage(pictureBox1.Image);


for (int i = 0; i < img1.Width; i++)
{
for (int j = 0; j < img1.Height; j++)
{
img1_ref = img1.GetPixel(i, j).ToString();
img2_ref = img2.GetPixel(i, j).ToString();
if (img1_ref != img2_ref)
{
count2++;
flag = false;
 
//drawing

x = i;
y = j;
pen = new Pen(Color.Red);
g.DrawEllipse(pen, x, y,100,100);


//drawing
MessageBox.Show("error");

break;
}
count1++;
}
progressBar1.Value++;
}
 
if (flag == false)
MessageBox.Show("Sorry, Images are not same , " + count2 + " wrong pixels found");
else
MessageBox.Show(" Images are same , " + count1 + " same pixels found and " + count2 + " wrong pixels found");
}
else
MessageBox.Show("can not compare this images");
this.Dispose();
}