There is more than one No.6 in the picture. I will not be surprised if the missing person in the picture is Baltar- a different version of him. Perhaps a Baltar that has come to terms with his true cylon identity and is confident.
Here is my theroy abt Baltar
Baltar is the Master Cylon and 1 of the Final 5 that ensures the cycle of "this happened before and will happen again". The Final 5 were not recently created but have always existed and undergo a continuous cycle of death and rebirth. They lose the memory of who they are each time and they require special triggers to realise their true identity. Because Baltar knows he will not rememeber what he is after he is reborn, "Original Baltar" programmed the 7 cylon models, including No. 6 to seek "Current Baltar" out and carry out his will - which is to cause the destruction of the 12 colonies and follow/guide Humanity to Earth. No. 6 in Baltar's head is actually a representation/projection of Baltar's own subconsciousness that helps to guide Baltar in his actions. This explains why Baltar is able to seemingly make random but correct guesses of where the cylon weak spot is (Hand of God), who is a cylon (pre-Season 1 movie), what is going to happen next (cylon-hybrid child), what is cylon equipment (cylon transponder), etc the list goes on when you watch it all again.