two ways to copy a System.Drawing.Bitmap into a 1D rgb pixel array.

gistfile1.txt

Bitmap bmpOrig = new Bitmap(@"C:\Users\Thomas\Desktop\picture.png");
Bitmap bmpResized = new Bitmap(bmpOrig, new System.Drawing.Size(640, 480));
Bitmap bmp = bmpResized.Clone(new Rectangle(0,0,bmpResized.Width,bmpResized.Height), System.Drawing.Imaging.PixelFormat.Format24bppRgb);
  //byte[] bytes = new byte[bmp.Width * bmp.Height * 3];
  //int index = 0;
  //for (int i = 0; i < bmp.Height; i++)
  //{
  //    for (int j = 0; j < bmp.Width; j++)
  //    {
  //        System.Drawing.Color color = bmp.GetPixel(j,i);
  //        bytes[index] = color.R;
  //        bytes[index + 1] = color.G;
  //        bytes[index + 2] = color.B;
  //        index += 3;
  //    }
  //}

  System.Drawing.Rectangle rect = new System.Drawing.Rectangle(0, 0, bmp.Width, bmp.Height);
  System.Drawing.Imaging.BitmapData bmpData =
      bmp.LockBits(rect, System.Drawing.Imaging.ImageLockMode.ReadOnly, bmp.PixelFormat);

  int bytes = Math.Abs(bmpData.Stride) * bmp.Height;
  byte[] rgbValues = new byte[bytes];

  IntPtr ptr = bmpData.Scan0; // Get the address of the first line.
  System.Runtime.InteropServices.Marshal.Copy(ptr, rgbValues, 0, bytes);
  bmp.UnlockBits(bmpData);

  string result = String.Join(",", rgbValues);
  System.IO.File.WriteAllText(@"C:\Users\Thomas\test2.csv", result);