viagra increases the nitric oxide levels in the blood, which in turn incrases blood flow to the smooth muscles (the involuntary ones).
your penis tissue and its abillity to fill with blood is controlled by smooth muscles (the penis itself is not a muscle, but the mechanism by which it fills is controlled by smooth muscle).
the only way I can think of it helping in posing is that the arterial walls constrict and/or dilate and this is controlled by smooth muscle tissue, so in theory it would make you look more veiny I suppose... in practice I've never noticed it - although it is probably b/c I'm too focuses on my rock hard cock.