Artist Chris Rahn has illustrated forty-six Magic cards, although that number's going to go up once Magic 2012 comes out.
The first step of the creation of a card's art is its art description. The art description tells the artist what the card will do, what its flavor is, and what the mood of the illustration should be. Here were the instructions Chris was given for Pride Guardian:
Color: White creature
Location: Savannah area, in front of a gate or large ornate door of a shrine or chapel.
Action: Show a leonin (lion humanoid) warrior in a defensive/guarding pose. His job is to prevent any unwanted entry into this sacred place. He wears light ceremonial garb, perhaps some combination of leather and linen (feels ceremonial in detail, but he can still fight in it) and wields a quarterstaff that is carved or adorned in a ceremonial fashion as well.
Focus: The leonin monk
Mood: A religious figure who could deflect any of your attacks
With that art description to work with, Chris submitted a sketch to the Magic creative team:
The creative team liked it, especially the hint of some sort of magic face paint on the forehead, so they gave Chris the go-ahead.
Here's the final card as you'll see it in Magic 2012 booster packs: