Ever since I first played BG2 I was always wondering who the two dudes on the front were. Do the have anything to do with BhaalSpawn or something? Or are they their to make the game cover look cool?

<font color="gold">To me, it looks like they are both the same dudes, they just show different aspects of him.
I also believe that it is supposed to be Balduran, the pirate who founded the city Baldur's Gate. ;) </font>

spoiler kinda
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i allways thought it was you, the bhaalspawn, like both aspects of you. you have a dark side, but also light (that is, if you arent a evil pc).

Well, I don't think it's you, I think it just shows like the yin and yang of everyone that exists in the realms, both good and evil like Xero said.

Top Half: Suldenessellar Elf
Bottom Half: Drow Elf

Has almost nothing to do with the Bhaalspawn in particular, but gives a nice sense of the Big Picture. All you are is the icing on the cake, running around doing fancy stuff like killing dragons.

These are the heads of the Child of Bhaal - your character.
Obviously, he has only one head, so this is some kind of a symbol of his choice: to be good and battle his blood, or to be evil and relinquish to it.
It was inspired by constant battle between Good and Evil.

And what Six says makes sense. Is it official explanation?

Damn, Six beat me too it. Yeah, its Suldanessar and Drow. At first, I thought it was the queen herself, but then i realized drow and suldanessar. And it makes sense.