var
  Target: TTargaGraphic;
{...}
          Target := TTargaGraphic.Create;
          Target.PixelFormat := pf24Bit;
          Target.Width := 256;
          Target.Height := 256;
          Target.Canvas.Draw(0, 0, Img1.Picture.Graphic);
           Target.SaveToFile('путь сохранения');
          Target.Free;

Target.Canvas.Draw(0, 0, Img1.Picture.Graphic);

procedure TMainForm.TruevisionTarga1Click(Sender: TObject);

var
  Target: TTargaGraphic;

begin
  with SPD do
  begin
    Filter := FileFormatList.GetGraphicFilter([], fstBoth, [foCompact, foIncludeAll,
     foIncludeExtension],TTargaGraphic);

    if Execute then
    begin
      Target := TTargaGraphic.Create;
      try
        if Image1.Picture.Graphic is TBitmap then Target.Assign(Image1.Picture.Graphic)
                                             else
        begin
          Target.PixelFormat := pf24Bit;
          Target.Width := Image1.Picture.Width;
          Target.Height := Image1.Picture.Height;
          Target.Canvas.Draw(0, 0, Image1.Picture.Graphic);
        end;
        Target.SaveToFile(FileName);
      finally
        Target.Free;
      end;
    end;
  end;
end;

BitBlt(Target.Canvas.Handle, 0, 0, 256, 256, img1.Canvas.Handle, 0, 0, SrcCopy);

BitBlt(Target.Canvas.Handle, 0, 0, 256, 256, bmp.Canvas.Handle, 0, 0, SrcCopy);