I'm resizing some images to the screen resolution of the user; if the aspect ratio is wrong, the image should be cut. My code looks like this:

protected void ConvertToBitmap(string filename)
    {
        var origImg = System.Drawing.Image.FromFile(filename);
        var widthDivisor = (double)origImg.Width / (double)System.Windows.Forms.Screen.PrimaryScreen.Bounds.Width;
        var heightDivisor = (double)origImg.Height / (double)System.Windows.Forms.Screen.PrimaryScreen.Bounds.Height;
        int newWidth, newHeight;

        if (widthDivisor < heightDivisor)
        {
            newWidth = (int)((double)origImg.Width / widthDivisor);
            newHeight = (int)((double)origImg.Height / widthDivisor);
        }
        else
        {
            newWidth = (int)((double)origImg.Width / heightDivisor);
            newHeight = (int)((double)origImg.Height / heightDivisor);
        }

         var newImg = origImg.GetThumbnailImage(newWidth, newHeight, null, IntPtr.Zero);
        newImg.Save(this.GetBitmapPath(filename), System.Drawing.Imaging.ImageFormat.Bmp);
    }

In most cases, this works fine. But for some images, the result has an extremely poor quality. It looks like the would have been resized to something very small (thumbnail size) and enlarged again.. But the resolution of the image is correct. What can I do?

Example orig image:

Example resized image:

Note: I have a WPF application but I use the WinForms function for resizing because it's easier and because I already need a reference to System.Windows.Forms for a tray icon.