Public Function PictureBoxImageToBytes(ByVal picBox As PictureBox) As Byte() 
If (picBox IsNot Nothing) AndAlso (picBox.Image IsNot Nothing) Then
Dim bmp As New Bitmap(picBox.Image)Dim ms As New System.IO.MemoryStream() 
bmp.Save(ms, System.Drawing.Imaging.ImageFormat.Bmp)
Dim buff As Byte() = ms.ToArray() 
ms.Close()

ms.Dispose()
Return buff 
Else

Return Nothing

End If

End Function

public byte[] PictureBoxImageToBytes(PictureBox picBox) 
{
     if ((picBox != null) && (picBox.Image != null))
    {
         Bitmap bmp = new Bitmap(picBox.Image);
         System.IO.MemoryStream ms = new System.IO.MemoryStream();

         bmp.Save(ms, System.Drawing.Imaging.ImageFormat.Bmp);
           
           byte[] buff = ms.ToArray();

         ms.Close();
         ms.Dispose();
         return buff;
    }
     else
    {
         return null;
    }
}

type
  TBytesDynArray = array of byte;
procedure BitmapToBytesArray(Bitmap: TBitmap; var BytesArray: TBytesDynArray);
var pX : ^byte;
    N,LZ : integer;
begin
  case Bitmap.PixelFormat of
    pf8bit : LZ := (Bitmap.Width * Bitmap.Height);
    pf16bit: LZ := (Bitmap.Width * Bitmap.Height) * 2;
    pf24bit: LZ := (Bitmap.Width * Bitmap.Height) * 3;
    pf32bit: LZ := (Bitmap.Width * Bitmap.Height) * 4;
  end;
  pX := Bitmap.ScanLine[Bitmap.Height-1];
  SetLength(BytesArray, LZ);
  CopyMemory(@BytesArray[0], pX, LZ);
end;

exemple :
var
  BA : TBytesArray;
begin
  BitmapToBytesArray(ImageX.Picture.Bitmap, BA);
  ...
end;

Type
  TBArray = Array of Byte;
Var aBArray : TBArray;
procedure Bitmap2ByteArray(const aBMP: TBitmap;var ByteArray: TBArray);
var MS: TMemoryStream;
begin
  MS := TMemoryStream.Create;
  try
    aBMP.SaveToStream(MS);
    MS.Seek(0, soFromBeginning);
    SetLength(ByteArray, MS.size);
    MS.ReadBuffer(ByteArray[0], Length(ByteArray));
  finally
    MS.free;
  end;
end;

// Utilisation
procedure TForm1.Button1Click(Sender: TObject);
begin
  Bitmap2ByteArray(Image1.Picture.Bitmap, aBArray);
end;

Private Type BITMAP
    bmType       As Long
    bmWidth      As Long
    bmHeight     As Long
    bmWidthBytes As Long
    bmPlanes     As Integer
    bmBitsPixel  As Integer
    bmBits       As Long
End Type
Private Declare Function GetObject Lib "gdi32" Alias "GetObjectA" (ByVal hObject As Long, ByVal nCount As Long, lpObject As Any) As  Long
Private Declare Function GetBitmapBits Lib "gdi32" (ByVal hBitmap As Long, ByVal dwCount As Long, lpBits As Any) As  Long
Function PictureToArray(ByRef img As IPicture) As Byte()
    Dim tBM     As BITMAP
    Dim abRet() As Byte
    Dim lSize   As Long
'    structure
    Call GetObject(img.handle, Len(tBM), tBM)
'   préparation du tableau
    lSize = (tBM.bmWidth * 3) And &HFFFFFFFC
    ReDim abRet(0 To lSize * tBM.bmHeight * 3)
'   attribution,  retour
    Call GetBitmapBits(img.handle, UBound(abRet), abRet(0))
    PictureToArray = abRet
    Erase abRet
End Function