Why are the Boeing 747-8 engines on the wings spread apart?
That picture of the 747-200 you have with the engines in a pod together like a B-52 is a movie prop. http://www.airliners.net/photo/Boeing-747-236B/1070481/

All real 747s from the -100 through the -8 have their engines positioned like the first picture.

EDIT - Federico's response reminded me of another aspect of the engine position, and to answer your original question of why they're spread apart. Spacing between the 747s engines is important due to the need to maintain sufficiently clean airflow into the inlet. If the engines are too close together, they're essentially competing for the same amount of air, and at high power settings, the interaction between the two fans will disrupt their operation, potentially causing engine stall/surge. Additionally, the 747 uses thrust reversers, and the spacing between the engines helps ensure that the reverse flow from each engine doesn't impact an adjacent engine, for the same inlet distortion reason. 

I assume, though I don't know the details of this, that the B-52 can have them that close together because they've shown that the inlet geometry, airflow required by each engine, whatever other magic of fluid dynamics that I know nothing about, prevents the engines from distorting each other's airflow enough to cause an issue. The 747 engines require a lot more air, and generate a lot more thrust, and are potentially more sensitive to inlet disruption than the engines on the B-52. This last paragraph is mostly a guess on my part as I'm not an aerodynamicist, but it's inferred from issues we've had to deal with on the 747 and other related airplanes.