A little excerpt:
The key to attracting artists is to get a lot of players, and advertise that you're open to contribution. Statistically, a certain fraction of your players will be artists. It's possible that if your game is aimed at certain subcultures, they'll have disproportionately higher numbers of people interested in art (this may, for example, include the fantasy/sci-fi community). In order to keep artists, you need to keep them motivated. Your artists are not being paid, so motivation is the only thing keeping them around. The moment it stops being fun or rewarding, they're usually out the door.
And more advice you can read here.