为什么透明像素在我的图像中显示为黑色?

我正在将图像字节数组保存为缩略图。问题是我的图像中透明的背景色是黑色的。

以下是我的代码:

MemoryStream memoryStream = new MemoryStream(pbytImageByteArray);

System.Drawing.Image imgImageSource = System.Drawing.Image.FromStream(memoryStream);

double dblOrgnWidth = imgImageSource.Width;
double dblOrgnHeight = imgImageSource.Height;

double dblRatio = (dblOrgnWidth / dblOrgnHeight) * 100;

double dblScaledWidth = pintWidth;
double dblScaledHeight = 0;

dblScaledHeight = (dblScaledWidth / dblRatio) * 100;
System.Drawing.Bitmap bitmapImage = new System.Drawing.Bitmap(System.Convert.ToInt32(dblScaledWidth), System.Convert.ToInt32(dblScaledHeight));

bitmapImage.SetResolution(imgImageSource.HorizontalResolution, imgImageSource.VerticalResolution);
System.Drawing.Graphics graphics = System.Drawing.Graphics.FromImage(bitmapImage);
graphics.CompositingMode = System.Drawing.Drawing2D.CompositingMode.SourceOver;
graphics.CompositingQuality = System.Drawing.Drawing2D.CompositingQuality.HighQuality;
graphics.InterpolationMode = System.Drawing.Drawing2D.InterpolationMode.High;
graphics.SmoothingMode = System.Drawing.Drawing2D.SmoothingMode.HighQuality;            
ImageAttributes imageAttributes = new ImageAttributes();            
graphics.DrawImage(imgImageSource, new System.Drawing.Rectangle(0, 0, System.Convert.ToInt32(dblScaledWidth), System.Convert.ToInt32(dblScaledHeight)), 0, 0, System.Convert.ToInt32(dblOrgnWidth), System.Convert.ToInt32(dblOrgnHeight), System.Drawing.GraphicsUnit.Pixel);

MemoryStream outputMemoryStream = new MemoryStream();
bitmapImage.Save(outputMemoryStream, System.Drawing.Imaging.ImageFormat.Jpeg);
bitmapImage.GetThumbnailImage(System.Convert.ToInt32(dblScaledWidth), System.Convert.ToInt32(dblScaledHeight), null, IntPtr.Zero);
imgImageSource.Dispose();
bitmapImage.Dispose();
graphics.Dispose();
return outputMemoryStream.ToArray();