While I do like bringing Gramps in as a nice surprise, he is only my pick if I really can't pick anybody in the photo. We are told in Razor that the fifth is in shadow, looking for the light, that he/she hungers for redemption and will only get it in great pain.

And that doesn't really describe Joseph Adama, or Gaeta, or just about anybody not in the picture except perhaps Cain, who does need redemption. But Cain is not in shadow and unaware of her status. She's either dead and not a Cylon, or downloaded and aware of what she is. So my bet is it's somebody in the picture.