As mentioned they were created in-house by a very talented artist. Think the time needed for each drawing was close to 40 hours (the original images are of a higher resolution than those we use on the website). He also created the rest of the illustrations we have on the website, including the official avatars. At the time of launching the website, he did not want his name announced, if I remember I'll ask him if he might change his mind next time I talk to him.
Games Workshop, let me know if you're looking for someone that actually can draw for the illustrations in the books, and I'll send over his details
On a side note:
There were more people involved with creating the forum we have, all from frontend to backend developers and all did a good job combining their skills to create what we use today.