When I first saw the picture I thought it was the Prime Evil. I'm still unsure about how combining the essences of the 7 Great Evils would work out. At first thought, you might think that it would rebirth Tathamet, but I'm not so sure it would. Tathamet is a far greater entity than just what the Great Evils would create. Again, I have no real clue about how it all works, but I think that the Prime Evil and Tathamet are separate, and that Tathamet cannot be reborn.
However, as I've thought about it some more, I must think that this demon in the picture is Baal fighting the Horadrim and their apprentices. Besides the fact that it looks like Baal, I can only pick out 7 people in the picture, all with staves or wands, implying that they are mages. According to the Book of Cain
The first group of Horadrim was small. Some say the number was seven; some say the number was twelve. Both numbers might be correct, and it can be assumed that the mages brought with them escorts, apprentices, and others to assist on their mission.
If this depiction is indeed the Horadrim and Baal, then the person most forefront would most certainly be Tal Rasha. Unfortunately, the only official picture of Tal Rasha that exists (as far as I'm aware) is in the Diablo I manual, and it can't really be taken as canon. That said, we do have one hint. In both Diablo II and Diablo III, the helm of Tal Rasha's set is a horned mask, and the person that I suspect to be Tal Rasha is clearly wearing a horned mask. I'd say that that's proof enough for me to be pretty certain that the picture depicts Baal.
Diablo Manual Horadrim Picture: