I think where possible, people try to base it on existing work, as you said (releases for other platforms, ads, etc..)

Heroquest is based on the art for its board game.

I believe there was an ad for California Raisins.

I made one for Recca, where I just altered the art from the Soundtrack packaging.

No idea about Drac.  I think that one is actually completely someone's idea. 

I've actually never liked the artwork for Earthbound.  The japanese version is all red, so most NES repros do the same thing.  What I did, what took the same look, but make the red look more like a nebula.

In the case of Mike Ditka, I believe NGD had the official promo art.  Same for Happy Camper.


Al

The art from Ditka came from one of those Vidpro cards.