var
  Bmp: TBitmap;
  x1, y1: Integer;
  px: PRGBQuad;
begin
  Bmp := TBitmap.Create;
  try
    Bmp.Width := Image1.Width;
    Bmp.Height := Image1.Height;
    Bmp.PixelFormat := pf32bit;
    Bmp.HandleType := bmDIB;
    Bmp.IgnorePalette := True;

    for y1 := 0 to Bmp.Height - 1 do
    begin
      px := Bmp.ScanLine[y1];
      for x1 := 0 to Bmp.Width - 1 do
      begin
        px.rgbBlue := 0;
        px.rgbGreen := 0;
        px.rgbRed := 0;
        px.rgbReserved := 0;  // fully transparent
        Inc(px);
      end;
    end;

    //...

  finally
    Bmp.Free;
  end;
end;