In hind sight it appears that the pic places emphasis on relationships.
I now think the Six next to Tigh is the Six he has been intimate with. In the pic we also see the relationship between Helo and Athena, Kara and Anders, Baltar and the Six in his head, and Apollo 'seems' to be by himself. Apparently the empty seat between Apollo and Saul was meant for Ellen, the fifth Cylon. Her placement was meant to represent her relationship with Saul. Apparently her absence is due to her death. On a side note the distance between Adama and Roselin represents their relationship. Perhaps it illustrates Roselin's fear of getting close to anyone.
On http://www.spacewesterns.com/articles/60/ it shows a few alternative versions of this pic which shows the relationship between Lee and Kara, as well as the relationship between Baltar and Tyrol.
Another thought, they may have conscidered making Duala the fifth cylon. I say that because the gap between Tigh and Lee could have also been for Duala. It would have illustrated the relationship between Lee and Duala.It would have made more sense conscidering that in the original version of the pic, Lee was all by himself.