purtroppo non è uguale a photoshop, ma vedi se va

codice:
    private void ResizeAndSave(Stream imgStr, int Width, int Height, string FileName)
    {
        EncoderParameters eps = new EncoderParameters(1);
        long compression = 100;
        eps.Param[0] = new EncoderParameter(System.Drawing.Imaging.Encoder.Quality, compression);
        ImageCodecInfo ici = GetEncoderInfo("image/jpeg");
        System.Drawing.Image bmpStream2 = System.Drawing.Image.FromStream(imgStr);

        Bitmap img = null;
        Graphics gr = null;

        img = new System.Drawing.Bitmap(Width, Height);
        img.SetResolution(bmpStream2.HorizontalResolution, bmpStream2.VerticalResolution);
        gr = Graphics.FromImage(img);
        gr.InterpolationMode = System.Drawing.Drawing2D.InterpolationMode.HighQualityBicubic;
        gr.SmoothingMode = System.Drawing.Drawing2D.SmoothingMode.HighQuality;
        gr.PixelOffsetMode = System.Drawing.Drawing2D.PixelOffsetMode.HighQuality;
        gr.CompositingQuality = System.Drawing.Drawing2D.CompositingQuality.HighQuality;

        gr.DrawImage(bmpStream2, 0, 0, Width, Height);
        gr.Dispose();

        img.Save(FileName, ici, eps);
    }