i want to draw a circle on a image(any format bmp,jpeg etc.)

SIR,
i am doing a application for image processing where i am comparing 2 immages,,and i want to highlight the pixcles where the diffrence occours with a red colored circle..i am able to compare the images but not able to highlight it with the circle..i want the pixel which is diffrent to be 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();
}