Create thumbnail without stretching/shrinking the image in c#

Question

I have this code that creates me thumbnails for my project:

//****  INICIO GUARDA THUMB  **********

// valores para altura e largura do thumb:
int resizeToWidth = 260; //px
int resizeToHeight = 260; //px

//busca-se a main image
Image thumb = Image.FromFile(saveFilePath, false);

//transforma-se a imagem para o tamanho pretendido
Bitmap bmp = new Bitmap(resizeToWidth, resizeToHeight);
Graphics graphic = Graphics.FromImage(bmp);
graphic.InterpolationMode = InterpolationMode.HighQualityBicubic;
graphic.SmoothingMode = SmoothingMode.HighQuality;
graphic.PixelOffsetMode = PixelOffsetMode.HighQuality;
graphic.CompositingQuality = CompositingQuality.HighQuality;
graphic.DrawImage(thumb, 0, 0, resizeToWidth, resizeToHeight);

//Aqui já temos uma Image que é um thumbnail
Image imageToSave = bmp;

//caminho fisico onde guarda-se o thumb
string thumbPath = Server.MapPath("~/thumbnails");

//caminho dentro do projeto onde guardam-se as thumbs
imageToSave.Save(thumbPath + "\\" + imgGuid + extension);

//******  FIM GUARDA THUMB  ***********

The imgGuid is a variable with the name of the image. The problem with this code is that if the image is very different from a square, it is all disfigured.

How can I use the C# to improve the algorithm and cut the excesses instead of shrinking them?

EDIT: Before using the answer I was given, I used this code to reduce the image size first, before cutting it:

if (thumb.Size.Height > thumb.Size.Width || thumb.Size.Height == thumb.Size.Width)
{
 thumb = ResizeImage(sourceImage, resizeToWidth, thumb.Size.Height);
}
else 
{ 
 thumb = ResizeImage(sourceImage, thumb.Size.Width, resizeToHeight);
}

public static Image ResizeImage(Image sourceImage, int maxWidth, int maxHeight)
        {
            //Calculam-se as dimensões novas para a imagem
            var ratioX = (double)maxWidth / sourceImage.Width;
            var ratioY = (double)maxHeight / sourceImage.Height;
            var ratio = Math.Min(ratioX, ratioY);
            var newWidth = (int)(sourceImage.Width * ratio);
            var newHeight = (int)(sourceImage.Height * ratio);

            //Cria-se um bitmap com as dimensões otimizadas
            var newImage = new Bitmap(newWidth, newHeight);
            //Desenha-se a imagem no bitmap
            using (var grafico = Graphics.FromImage(newImage))
            {
                grafico.InterpolationMode = InterpolationMode.HighQualityBicubic;
                grafico.SmoothingMode = SmoothingMode.HighQuality;
                grafico.PixelOffsetMode = PixelOffsetMode.HighQuality;
                grafico.CompositingQuality = CompositingQuality.HighQuality;
                grafico.DrawImage(sourceImage, 0, 0, newWidth, newHeight);
            };

            return newImage;
        }

Answer 1

would say to do a Crop.. here at this link below (code) shows how.

https://stackoverflow.com/questions/734930/how-to-crop-an-image-using-c

       Rectangle cropRect = new Rectangle(...);
       Bitmap src = Image.FromFile(fileName) as Bitmap;
       Bitmap target = new Bitmap(cropRect.Width, cropRect.Height);

       using(Graphics g = Graphics.FromImage(target))
       {
         g.DrawImage(src, new Rectangle(0, 0, target.Width, target.Height), 
                cropRect,                        
                GraphicsUnit.Pixel);
       }