Lorraine Mehve R. McLees posted in the HBO forum the other day, just to clarify a couple of comments people were making on the Official Xbox Magazine cover art. The work is, indeed, hers-- but she was so tired after finishing it that she forgot to sign it.
She pointedly decided NOT to clarify whether or not the figures standing next to the Master Chief on that cover are merely marines or other Spartans. However, anyone who's seen any of the recent trailer footage (who, us?) may note that the marines in it are wearing green armor not unlike the Chief's, which may go a long way towards explaining the Spartan-like appearance of the other figures on the cover.