Magnify and change contrast of image

Yes i did, I am following this link:
http://www.switchonthecode.com/tutorials/csharp-tutorial-convert-a-color-image-to-grayscale
public static Bitmap MakeGrayscale(Bitmap original)
{
//make an empty bitmap the same size as original
Bitmap newBitmap = new Bitmap(original.Width, original.Height);

for (int i = 0; i < original.Width; i++)
{
for (int j = 0; j < original.Height; j++)
{
//get the pixel from the original image
Color originalColor = original.GetPixel(i, j);

//create the grayscale version of the pixel
int grayScale = (int)((originalColor.R * .3) + (originalColor.G * .59)
+ (originalColor.B * .11));

//create the color object
Color newColor = Color.FromArgb(grayScale, grayScale, grayScale);

//set the new image's pixel to the grayscale version
newBitmap.SetPixel(i, j, newColor);
}
}

return newBitmap;
}
I AM PASSING VALUES TO THIS METHOD AS:

protected void grayscale_Click(object sender, EventArgs e)
{
//Get the path of image file

string path = Image1.ImageUrl;

//create bitmap from path and pass as argument to function makegryscale2

Bitmap b = MakeGrayscale(new Bitmap(path.Substring(2)));

//overwrite with saving at same path

b.Save(path);

//set it imageurl

Image1.ImageUrl = path;

}
BUT it is giving me an error, the error says Invalid Parameter. plz let me know how can i use this method????
Also, can u provide me link of ur articles??