try this
public static Image ScaleImage(Image image, int maxWidth, int maxHeight)
{
var ratioX = (double)maxWidth / image.Width;
var ratioY = (double)maxHeight / image.Height;
var ratio = Math.Min(ratioX, ratioY);
var newWidth = (int)(image.Width * ratio);
var newHeight = (int)(image.Height * ratio);
var newImage = new Bitmap(newWidth, newHeight);
Graphics.FromImage(newImage).DrawImage(image, 0, 0, newWidth, newHeight);
return newImage;
}
Usage
Image img = Image.FromStream(stream);
Image thumb = ScaleImage(img);
stream.Close();
stream.Dispose();
stream = new MemoryStream();
thumb.Save(stream, System.Drawing.Imaging.ImageFormat.Png);