The simple answer, in a situation where there are really no simple answers is that copyright protects the expression of a work. In this case, you own and can decide what happens with your specific description of the work. If you have an artist render visual expressions of your text you enter the complicated world of joint copyright. The artist has copyright over his particular expression of the work, but the work as a whole is in all probability jointly owned by both parties. It all depends on how closely the final work represents your text expression. In your spear example, if you simply asked for a spear, and you were given your artists concept of a spear, the final work would unlikely be considered to have joint copyright.

Shades of Gray...

In this situation the simple solution is to have all the work done in a "Work for Hire" relationship. This is where you pay or compensate the person for the purpose that you will own the sole copyright over the work. The compensation is of course negotiable between the different parties, but should respect contract law. It could be as simple as having the artist's name appear in the credits section.

It's times like these that make me wonder what I was thinking when I decided to go to school for Law Clerk for the sole purpose of helping me in my failed attempt in getting into the RPG publishing industry....