procedure TImgViewerForm.LoadCR2Thumb(const AFileName: String);
var
  FS: TFileStream;
  MS: TMemoryStream;
  B: Byte;
  JPG: TJpegImage;
  c: Integer;
begin
  FS := TFileStream.Create(AFileName, fmOpenRead);
  FS.Seek($2600, soFromBeginning);

  MS := TMemoryStream.Create;
  while True do
  begin
    FS.Read(B, 1);
    if B = $FF then
    begin
      FS.Read(B, 1);
      Inc(c);
      if B = $D9 then
      begin
        c := FS.Position - $2600;
        Break;
      end;
    end;
    if (FS.Position = FS.Size) then
      Exit;
  end;
  FS.Seek($2600, soFromBeginning); // Ab hier geht dann alles ganz schnell. Nur die vorangehende Schleife ist super lahm.
  MS.CopyFrom(FS, c);
  MS.Seek(0, soBeginning);
  JPG := TJPEGImage.Create;
  JPG.LoadFromStream(MS);

  Img.Picture.Assign(JPG);
  JPG.Free;
  MS.Free;
  FS.Free;
end;

var
  Buffer : array of byte;
  f : file;

begin
  assignfile(f,Datei);
  setlength(Buffer,filesize(f));
  reset(f,1);
  Blockread(F,Buffer[0]);
  Closefile(f);
  // Suchen...
end;