Delphi_Image_Resize

procedure ResizeIm(Original, Output : TBitmap; W, H : Integer);
var
  buffer: TBitmap;
begin
  buffer := TBitmap.Create;
  try
    buffer.SetSize(W, H);
    buffer.Canvas.StretchDraw(Rect(0, 0, W, H), Original);
    Output.SetSize(W, H);
    Output.Canvas.Draw(0, 0, buffer);
  finally
    buffer.Free;
  end;
end;

procedure TForm1.FormResize(Sender: TObject);
var
  W, H : integer;
begin
  if (ScrollBox1.Width < OriginalImage.Width) or (ScrollBox1.Height < OriginalImage.Height) then
  begin
    if (OriginalImage.Height > ScrollBox1.Height) and (OriginalImage.Width > ScrollBox1.Width) then
    begin
      if ScrollBox1.Height > ScrollBox1.Width then
      begin
        H := ScrollBox1.Height - 10;
        repeat
          H := H - 10;
          W := Round (h * (OriginalImage.Width / OriginalImage.Height));
        until W < ScrollBox1.Width - 10;

        ResizeIm(OriginalImage, Image1.Picture.Bitmap, W, H);
      end
      else
      begin
        W := ScrollBox1.Width - 10;
        repeat
          W := W - 10;
          H := Round (w * (OriginalImage.Height / OriginalImage.Width));
        until H < ScrollBox1.Height - 10;

        ResizeIm(OriginalImage, Image1.Picture.Bitmap, W, H);
      end;
      exit;
    end;
    if (OriginalImage.Height > ScrollBox1.Height) then
    begin
      H := ScrollBox1.Height - 10;
      W := Round (h * (OriginalImage.Width / OriginalImage.Height));
      ResizeIm(OriginalImage, Image1.Picture.Bitmap, W, H);
      exit;
    end;
    if (OriginalImage.Width > ScrollBox1.Width) then
    begin
      W := ScrollBox1.Width - 10;
      H := Round (w * (OriginalImage.Height / OriginalImage.Width));
      ResizeIm(OriginalImage, Image1.Picture.Bitmap, W, H);
      exit;
    end;
  end
  else
  begin
    Image1.Picture.Assign(OriginalImage);
  end;

end;