//Obtain original image from input stream
using (var sourceImage = new Bitmap(Image.FromStream(inStream)))
{
    //Thumbnail size and quality that may be saved in app settings or an environment variable
    int thumbSize = 150;
    int thumbQuality = 75;

    //Obtain source dimensions and initialize scaled dimensions and crop offsets
    float sourceWidth = sourceImage.Width;
    float sourceHeight = sourceImage.Height;
    float scaledSourceWidth = thumbSize;
    float scaledSourceHeight = thumbSize;
    float offsetWidth = 0;
    float offsetHeight = 0;

    //Calculate cropping offset
    if (sourceWidth > sourceHeight)
    {
        offsetWidth = (sourceWidth - sourceHeight) / 2;
        scaledSourceWidth = (sourceWidth / sourceHeight) * thumbSize;
    }
    else if (sourceHeight > sourceWidth)
    {
        offsetHeight = (sourceHeight - sourceWidth) / 2;
        scaledSourceHeight = (sourceHeight / sourceWidth) * thumbSize;
    }

    //Create new thumbnail image of height and width defined in thumbSize
    Bitmap thumbnail = new Bitmap((int)thumbSize, (int)thumbSize, sourceImage.PixelFormat);
    thumbnail.SetResolution(sourceImage.HorizontalResolution, sourceImage.VerticalResolution);

    using (var graphics = Graphics.FromImage(thumbnail))
    {
        //Draw source image scaled down with aspect ratio maintained onto the thumbnail with the offset
        graphics.CompositingQuality = CompositingQuality.HighSpeed;
        graphics.InterpolationMode = InterpolationMode.HighQualityBicubic;
        graphics.CompositingMode = CompositingMode.SourceCopy;
        graphics.DrawImage(sourceImage, new Rectangle(0, 0, (int)scaledSourceWidth, (int)scaledSourceHeight), offsetWidth, offsetHeight, sourceWidth, sourceHeight, GraphicsUnit.Pixel);

        //Push thumbnail onto stream for upload
        using (MemoryStream stream = new MemoryStream())
        {
            var encoderParameters = new EncoderParameters(1);
            encoderParameters.Param[0] = new EncoderParameter(Encoder.Quality, Convert.ToInt64(thumbQuality));
            var codecInfo = ImageCodecInfo.GetImageDecoders().FirstOrDefault(c => c.FormatID == ImageFormat.Jpeg.Guid);
            thumbnail.Save(stream, codecInfo, encoderParameters);
            stream.Position = 0;

            //Upload thumbnail to storage or download as an image file
        }
    }
}