Get GIF Dimensions

type
  TImageSize = record
    Width: Integer;
    Height: Integer;
end;

//

function ReadGIFSize(Stream: TStream): TImageSize;
type
  TGifHeader = record
    Signature: array [0..5] of Char;
    Width, Height: Word;
  end;
var
  Header: TGifHeader;
begin
  FillChar(Header, SizeOf(TGifHeader), #0);
  Result.Width := -1;
  Result.Height := -1;
  with Stream do
  begin
    Seek(0, soFromBeginning);
    ReadBuffer(Header, SizeOf(TGifHeader));
  end;
  if (AnsiUpperCase(Header.Signature) = 'GIF89A') or
    (AnsiUpperCase(Header.Signature) = 'GIF87A') then
  begin
    Result.Width  := Header.Width;
    Result.Height := Header.Height;
  end;
end;

procedure TForm1.Button1Click(Sender: TObject);
var
  fs: TFileStream;
  gifsize: TImageSize;
begin
  if OpenDialog1.Execute then begin
  fs := TFileStream.Create(OpenDialog1.FileName, fmOpenRead or fmShareDenyWrite);
  try
    gifsize := ReadGIFSize(fs);
    ShowMessage(Format('Breite %d Höhe %d', [gifsize.Width, gifsize.Height]));
  finally
    fs.Free;
    end;
  end;
end;