Melandrhild: Wow, thanks a lot ! I found some others but this one is the best I've seen :)
brozo: You are welcome.
According the credits in the manual the artist for the box illustration is someone called L.M. Jones. I tried googling to see if I could refine it further but without a first name too many probable false hits.
Correct. The cover art is from an original painting in oils by Larry M. Jones created specifically for the purpose. The original was fairly substantial in size. Some photographic references were used during its creation. It is known that the original was auctioned off on eBay about 10 1/2 years ago in the Spring of 2013 by its owner at the time, who had originally purchased it directly from the artist. So far as I know the identity of the auction winner has ever been publicly disclosed.
The information given here comes from contemporary posts to the "Darklands Discussion Group", at the time hosted on Yahoo!Groups, but migrated full and intact to Groups.IO (where it can still be found today) when the former service was discontinued in late 2019. The seller of the art was at the time a member of the group and had reached out to other members to raise visibility for the auction and measure interest.
Cheers!