King Conan said:
Why not use cover art from the novels, maybe something like Conan fighting a giant serpent? Please use comic strip art for your interiors - nobody can beat John Buscema! (before somebody chips in to tell me he's dead, I know. Use old Marvel Comics artwork - they adapted RH Howard story so you have lots of quality stock illustrations to use.
The problem with trying to use previous art work for a new book is in getting the rights to use that material. With several past artists, all of whom have had their art published in one format or another, first you need to track down who currently owns the rights to that art. Then you need to see if the rights that they have allows them to give any permission for another company to use that artwork. Some copyrights allow a company to continue to publish the art only in their own in-house publications.
Also, since this is a new version of the Conan rpg, it seems only correct that it should include new art. Mongoose wants this game to standout from any prior "barbarian" based rpgs, to use the art from past material would make this game seem to be nothing more then a carry on from the old TSR or GURPS products.
As with any new development, there will be hits and misses. From everything that I've seen and read about this new game, there are alot more hits then misses.