Although I am no expert on the "Last Supper" painting, my understanding is that the person "missing" in the BSG portrait is in the position of Judas (relative to DaVinci's painting).
Although Ron M. has to do his best to keep us guessing, when asked if the final cylon is the one missing from the photo, he doesn't reallly confirm or deny the statement. He has to give as ambiguous of an answer as possible.
If you consider that three of the four (of the five) are in the photo (Saul, Anders and Chief), where is Tori (Tory?)? It strikes me that Tori, if anyone, is so far diong the work of Judas in the series. She has already betrayed Chief by killing his wife. And she will probably have another betrayal role at some point (she has been working covertly behind the scenes with the president - tick tick tick - and is also bedding with Baltar - tick tick tick). So if you fill the gap with Tori as Judas, that means the final cylon IS in the picture. Which narrows the field down quite a bit assuming that Ron wouldn't leave the important cylon(s) out of the photo. Starbuck ... Roslin ... Baltar ... all are possible. "And the fifth, still in shadow, will claw toward the light, hungering for redemption that will only come in the howl of terrible suffering". I would say that both Baltar and the president are suffering and hungering for redemption to as great a degree as Starbuck. But then, if you go with the uber-surprise ending that some (including me) speculate could happen, there could be a thirteenth cylon that even the twelve don't know about. The thirteenth could be the "Angel" that the hybrid refers to. This could mean two of Roslin OR Starbuck OR Baltar all have an equal opportunity to bring up the rear as cylons. Put Starbuck in as #12 and Roslin in as #13 (the Angel that will deliver them) and then have Tori (Judas) kill the president, and you have the last supper. I know ... I know ... Six is in the position of Jesus ... But the "Six" in the middle is Baltar's psyche. Since she keeps telling him (in his mind) that God has a greater plan for him, and he has been instrumental in so many important points in the series, maybe he is supposed to be #13 and the deliverer of the people to salvation. After all, when Baltar was on the base ship, with the long hair and the white robe, wasn't it just a little too creapy of a Jesus image for those episodes? And he is the only character that has as complete an understanding of both sides - cylon and human. So who better to bring them together?
I say Starbuck as #12 and Baltar as the surprise #13.
I take back my blog a few months ago about Doc Cottle as #12. I was just trying to be different.
Ciao.